No.
A druid can only wildshape on his turn. So, while he can apply unlimited hitpoints to his form by wildshaping every turn, he cannot prevent you from dealing damage when it isn't his turn. If the druid is knocked out of his form (reduced to 0 hp), the first thing that happens is the excess damage carries over. But now he's essentially vulnerable until his turn. That's where the real damage comes in. You can think of wildshape as a regenerative shield of hit points.
Party Versus NPC Archdruids
There are a multitude of ways to "beat" the druid, just like any other encounter. Use your wits, bargain, stealth, combat, etc etc. Those tools are ALWAYS available. Your DM should be ready for you to avoid combat if you wish.
Party Versus PC Archdruids
If you're talking about PC vs PC combat, there are also a plethora of spells that can assist in killing the druid. For instance, if the druid has less than 100 HP in wildshape OR not, a Warlock can cast Power Word: Kill, and the druid simply dies. No save, just dead. That's just one spell. There are tons of others. A Druid can't wildshape if they're incapacitated. They, again, also can't wildshape unless it is their turn, so plan your heavy attacks accordingly. When a druid is knocked out of form, they are vulnerable, so maximize your damage in that time or make that vulnerability last longer with controlling spells. Any damage that exceeds a druid's wildshape form is applied to the druids regular form. Druids are very good at having health, so find a way to attack them in ways where health isnt the primary objective. Use conditions, spell effects, and any means to hinder the druid and he/she shouldn't be that tough of a fight.
Specialization
DnD 5e rewards specialization. An Archdruid specializes in wildshape to effectively gain temporary hitpoints among other combat abilities. It's the whole point of a wildshaping druid. Being a brown bear is cool, but when you can wildshape into one and get its free hitpoints, it becomes a big deal. That's largely what wildshaping druids are meant to do, so it only makes sense that it gets ridiculously good at it at level 20. All classes have a 20th level ability they get that helps them be even more specialized. It's a reward for forgoing multiclassing. Therefore, it should feel hard to deal damage to an Archdruid. But it's not impossible.
The first option - if the target makes a save, your reaction has already been used.
I agree that the sentence by itself is ambiguous. However, from my experience, any feature in 5e that forces a save and requires some kind of action consumes the action regardless of the result of the save. Therefore, I believe the same thing is intended here.
Notably, there are some features that you can choose to use after you know if you succeed, but afaik they all depend on you hitting with an attack.
Regardless, think of the Halo of Spores in-universe: you're using your reaction to blast the opponent with your spores, therefore your reaction is used regardless of the save. You don't ask your opponent "Hey, would you like to get blasted with spores? Yes? Well, here you go!", using your reaction only if he agrees/fails to defy you ;)
Best Answer
First, about the feature's wording.
The wording of the Symbiotic Entity feature that you quoted doesn't completely match either the Unearthed Arcana version of the Circle of Spores or the later published Guildmaster's Guide to Ravnica version, although it is very similar to both.
Here's the version from Unearthed Arcana: Three Subclasses, where it first appeared:
And here's the Guildmaster's Guide to Ravnica version (this is fair use):
So you might want to check the source you got your quote from to make sure you have accurate versions of this and other subclass features you use. Your quote looks like it was made by someone updating the UA version with the changes in the Ravnica version without quite matching the wording, and that could be problematic if changes were overlooked anywhere else in the subclass.
Regardless, the same key wording ("rather than transforming") is used in your quote, in the UA version and in the Ravnica version, so this answer is effectively universal. You might want to check the source you got your quote from, however, to make sure you have accurate versions of all your subclass features.
Yes, you ignore the downsides.
If you use your Wild Shape feature to do something else "rather than transforming" then you are literally not transformed after that, opting for another benefit in its place, in this case awakening your spores. Thus any restrictions you would normally be subjected to "while you are transformed" would literally not apply.
Note the following wording in the druid's Wild Shape feature (emphasis mine):
This precedes the list of benefits and restrictions you are subjected to when you are in a beast form, including the restriction of being unable to cast spells. These would not apply unless you're actually transformed.
If they were intended to apply just because you used the Wild Shape feature even if it didn't involve transforming, then it would say "while using this feature" or "while benefiting from this feature" rather than "while you are transformed." (And the designers would have been unreasonable to publish the Circle of Spores with the wording used if there were such an ambiguity.)
Effectively, this allows the Circle of Spores feature to utilize your Wild Shape uses as an expendable resource without creating a new resource just for that purpose (the Ravnica version of the feature makes this explicit) and without stacking the benefits of awakening your spores on top of the benefits of being in beast form.