Hydration numbers aren't that meaningful by themselves -- whether an 80% hydration level can produce a high-rising free-form loaf will depend on a lot on the types of flours or grains that are used. (Usually, 80% hydration is most appropriate for flatter or roughly shaped breads: ciabatta, focaccia, pizza dough, rustic baguettes, etc.) With the specific mixture of spelt and "strong white flour" (high protein) you mention, it should be possible to get a loaf with a little more lift. But it's hard to say for certain -- the flour itself will affect whether it's actually possible to do what you want.
Frankly, there are a lot of variables that could be creating problems beyond the ingredients. A sourdough culture that produces a lot of acid quickly can make it quite difficult to get a tall loaf. Or, if your sourdough yeast is weak and takes more than a couple hours for each rise, you might be producing too much acid. The acidic environment will tend to weaken the gluten, and you'll inevitably get a loaf that spreads. If this is the problem, you'll need to refresh the starter with a few closely spaced builds that really dilute the starter (e.g., dilute your starter 1:4 or even more with new flour/water in each build). That will strengthen the yeast but cut down on the early development of acidity. Unless you're a sourdough expert, I might actually suggest trying to get good results with regular baker's yeast in your recipe before doing the sourdough conversion, since the sourdough may be contributing more than anything else to the spreading.
If the starter isn't the problem and the ingredients can hold up the loaf, the next options are alterations to technique. The best suggestion I can give is to introduce "stretch-and-fold" maneuvers into the first rise. After you mix the final dough (which doesn't necessarily need to be heavily kneaded), come back every 30-45 minutes or so and stretch the dough from each side at a time. Pull out, lift up, and fold on top of the rest of the dough. Do this from each of the four sides of the dough. Let rest for 30-45 minutes again and repeat as often as you need until you feel the dough strengthen significantly.
If you adopt stretch-and-folds, you may not see the same amount of rise you saw without them, so you need to just keep your eye on the clock and use the same amount of time you did before for the first rise. By the time you reach the shaping phase, the dough should be much more taught and elastic.
The other significant issue is shaping. Do you do a pre-shape and bench rest before the final shaping? That can also help. Preshape by pulling the dough taught, folding in upon itself a number of times, then let rest for 15 minutes or so before doing final shaping. Basically, the more times you stretch the gluten and let it rest, the stronger the dough will get -- whether you do that in folding during the first rise or in a preshaping before bench rest, it will help. The shaping technique itself can also significantly affect the stability of the final loaf (but that's hard to explain in a text response).
Also, you may be waiting too long in the final proof if you wait to fully double in size. Unless the sourdough culture is very strong, you might have better results by waiting for only a 1.5 or 1.75 rise. If you do that, you may want to be somewhat gentle during the final shaping to preserve some of the gas from the first proof.
In fact -- if all of this sounds way too fussy for you, another option may be to skip all the stretching, folding, shaping, etc. and avoid the final rise almost altogether. Let the dough roughly double in size during the first rise, then shape very gently into a rough ball, rest just a short time until it starts to expand again (no more than an hour or so), and then throw it in to bake.
Lastly, the baking method could change things. If you're baking on a flat stone, the dough can just spread significantly during baking itself. I've sometimes seen this with high hydration sourdough -- I load it on a peel, and it looks okay before it goes in the oven, but during the oven spring phase, it just becomes wider instead of taller. Baking in a pot or round pan could help to restrain that spreading a bit without making it look too much like bread from a pan. And, if you cover a pre-heated pot for the first 15 minutes or so of the bake, it will help the oven spring and crust development.
It's hard to know which of these options will work best for you, since there are a large number of issues that could be at fault in this case. In my experience, however, the two most likely causes for spreading are a weak sourdough starter or inadequate strengthening of the gluten during shaping.
Best Answer
Note: This answer goes in a bit more detail than necessary to answer the question. If you truly only care about the hydration, please only read "Water/flour ratio" and "Flavor of the bread". I have added the other information as well since the effects are similar to that of a change in hydration.
I've frequently baked (about once to twice a week) with sourdough for 4 years now. I'm not a professional. The only thing I can take a bit of pride in is that a professional baker who tested a loaf of my sourdough bread told me that it's "fantastic, considering the little possibilities available to control the conditions the sourdough and loaf are exposed to at home". So while far from perfect, I'd consider my sourdough breads and knowledge pretty good.
Most of what I'm writing now was first read on the internet somewhere and confirmed through testing it with my own sourdough culture (I'm still using the same one I started out with).
Apart from the flour you use, there are several factors that affect the taste of the resulting bread:
Temperature
This has, in my experience, the biggest impact on the taste. The following rules apply:
Anything over 40°C is deadly to the microbes in your sourdough.
Lower temperature: More acetic acid, less lactic acid, yeasts develop more slowly.
Higher temperature: More lactic acid, less acetic acid, yeasts develop more quickly.
Lactic acid bacteria prefer temperatures at around 30-35°C
Yeasts like temperatures around 25°C best
Note: Your sourdough can be 1-2°C warmer than the environment due to microbial activity. That can be important to keep in mind.
Water/flour ratio
Most of the time, people use a ratio of 1:1 water/flour for the sourdough and that will give you good results, but you can play around with that ratio to change the taste to your liking:
Lower water content causes more acetic acid production and less developed yeasts
Higher water content will make it easier for the yeasts to grow and increase the lactic acid content
I've found that having more than 1.5 times the amount of water compared to the flour will hardly have any effect on the dough anymore and I'd recommend stirring the dough every couple of hours with that much water.
On single staged sourdough, using less than 0.75 times the water compared to the flour is also not a good idea and you will probably be required to add yeast in the bread dough.
Salt and oil
Adding these, especially salt, to the sourdough already will have a considerable impact on the sourdough taste. A sourdough made with salt in it will taste quite different from one without it. I'm not quite sure about why this is, but it may have something to do with the yeasts growing more slowly (more information below).
Both salt and oil will slow down the growth of the yeasts, but don't have much of an effect on the lactic acid bacteria.
Adding oil mainly affects the mechanical properties of the dough (it will be softer and more ductile). It also helps with the structure of the bread (the air bubbles in the bread will be smaller and more uniformly distributed), especially in the presence of an emulsifyer (lard is both fat and an emulsifyer, so it works well in this regard). For this reason, it doesn't make much sense to add it to the sourdough already.
Of course, if the fat isn't tasteless, it will also add to the taste of the bread (I love using natural olive oil in my wheat breads).
Flavor of the bread
Sourdough actually has some pretty complicated chemistry that I know little about. However, here are some effects that I do know:
Lactic acid will give your bread a mild, sour taste. This works very well for wheat breads.
Acetic acid gives you a stronger, more sour taste (very noticable while eating). This works great for rye breads
The amount of yeast in your sourdough also has a big impact on the smell and taste of the bread. Yeast is beneficial to the bread not only for it's leavening properties. Apart from CO2, it also produces ethanol, which will esterify the lactic acid to ethyl lactate over time, which in turn has a strong flavor.
Okay I think that's it. I hope this helps you.