No, 12 AWG conductors are not large enough for 30 amperes.
Powering a structure is a complex task, which would require more details than can be provided in a short form Q&A site like this. If you need help with a small portion of this project, you might find help here. As for planning the entire project, this is not the place for that.
My advice would be to hire a local licensed Electrician, even if only for the planning part of the project.
Ok you wrote a book. Proposing all manner of third rate hackery. And what does it boil down to? You want to get 5000W out of your 5000W generator. Quick question.
What is 240 x 21 ?
By my math, it's 5040. There's your 5000W. You do get it out of the big NEMA L14-20 connector.
I have no idea where you got 41A. I'm pretty sure you made that up, probably by dividing 5000 by 120. I seriously doubt it was on the generator spec. There's a way if you really really want that, but as you get educated, you will realize you do not.
What is it you're missing? The odd idiom of North American 2-pole service. I don't blame you for not getting it... It's weird.
Your house is served by +120V, neutral (0V), and -120V. I just described an instant in time, they're AC so they will reverse position 120 times a second. The poles are called L1 and L2 and the middle is Neutral.
240V loads grab L1 and L2. 120V loads grab either pole and neutral. Which pole they grab is nearly random and that's the idea, to make them average out so loads are balanced.
For you, with 21A on each pole, balancing is a big deal. You'll have a problem if you put 30A of load on one pole. So you'll need to get into the gory details of what is on which pole, and manage accordingly.
Step 1: Control MWBCs so they don't kill you
I don't recommend rearranging things on a panel because you can break a type of wiring called a multi-wire branch circuit. Find an electrician and tell him to do exactly this:
find every multi-wire branch circuit in my home, and make sure both its hot wires are served from the same 2-pole breaker.
Step 2: get rid of double-stuff breakers
If your panel is stuffed, and has lots of breakers that have 2 breakers in 1 space, those will drive you absolutely bat crazy. ack... You know what, to heck with all that.
Let's just get you a new subpanel with the appropriate interlocks, and move the loads you want the generator to power into this new subpanel. Make this subpanel quite large (at least 20 space) realizing you'll use 4 spaces just for the interlock.
In a perfect world, your new panel will have ammeters which will tell you how close to 21A each pole is getting. Even better get one of those new fangled whole house monitoring systems. Ask a new question on how to get one to work in a generator interlocked panel.
Step 3: rearrange your loads in the panel
Now finally, it's time to learn the gory details of how poles are assigned in a panel. Read my posting here. Your panel may differ, but probably not by much.
Move your loads into the new panel, and consciously and carefully balance the loads. For instance if your table saw is on L1, put your dust collector on L2. Stuff like that.
Best Answer
Sharing the EGC is fine
No matter how many circuits you run in a single raceway, you only need one equipment grounding conductor for the whole lot (unless you require redundant or isolated grounding conductors, of course). This is implied by 250.122(C):
For your use case, a single 8AWG EGC (adequate for all circuits up to 100A) is sufficient here. Note that this conductor must be a wire in the setup described below as it serves as the separately derived system's grounding electrode conductor as per 250.30(A)(4) and the Exception to 250.121 in addition to being the equipment grounding conductor for both circuits, and the ground wire from the transfer switch to the main panel must likewise be an 8AWG wire.
Derates aren't too big a deal here either
Given that we can guarantee that there are only 4 current-carrying conductors (generator L/L1, generator N/L2, landscape lights L, landscape lights N), we can operate at an 80% derate from the 90°C column in Table 310.15(B)(16), allowing 6AWG THHN to carry 60A, more than you will need for just about any generator you want to use.
Conduit sizing
Given all this, and your (not unreasonable) desire to use Schedule 80 PVC for this wiring run, our 6 conductors (3 6AWG THHNs to make sure we have enough room for a 240V generator hookup, an 8AWG bare ground for the lot, and 2 12AWG THHNs for the landscape lights) add up to 127mm2 of fill. A 1" Schedule 80 provides 178mm2 of fill, but I would go up to 1.25" here which gives us a nice, generous 320mm2 of fill in order to avoid having to call in a pro to rescue a botched pull job.
As to transfer setups
You are on the right track with breaking your standby loads out into a separate panel -- this allows the use of inexpensive interlocked-breaker type manual transfer switches instead of loadcenter interlock kits (which limit flexibility) or select circuit manual transfer switches (which are awkward and hokey). It also reduces the size of the equipment required compared to service entrance transfer setups (which are a far too common occurrence with ATSes).
Given all this, I would recommend either an Eaton CH10GEN5050(R)SN if you want a prepackaged solution, or the combination of a suitable main lug Eaton CH loadcenter with a matching "Type A" Eaton CH generator interlock cover, two Eaton CH350SW breakers for the interlocked mains, and Eaton CH branch breakers if you wish to build something up yourself and can get the CH350SW (its production status is a bit questionable at the moment). If neither of these are an option, a Reliance Controls XRR1006D(R) (or XRR1006C if you want wattmeters for the generator) with suitable 1" wide branch breakers (HOM, QP, or BR, but not THQL or CL) can be substituted instead. Of course, you'll need a suitable inlet box for plugging your generator in outside as well -- either a L5-20 inlet for 120V only, a L14-30 inlet for 240V 30A, or a CS6365/CS6375 inlet for 240V 50A.
What makes these three solutions special is that they provide a switched neutral between the utility and generator sides of the system, allowing the generator N-G bond to stay intact. Practically, this means that just about any generator out there can be used plug-and-play with a suitable generator cord instead of having to fiddle around with the generator to pull the neutral-ground bond out of it, as most generators ship with their neutral-ground bond installed for portable standalone use, such as on jobsites.
Other residential transfer setups do not switch the neutral, which requires pulling the neutral-ground bond from the generator to avoid a paralleled bond that can falsely trip GFCI or many AFCI breakers and pose a safety hazard due to current on grounding conductors.
Other notes
You'll need a preprinted warning placard at your inlet in accordance with NEC 702.7(C) that specifies whether your system uses a bonded-neutral or a floating-neutral generator, as follows:
or
depending on what you have. You will also need a label on your main panel specifying where the generator inlet is, as per 702.10(A) -- this could be incorporated into the panel directory, though. More importantly though, your main panel will need a preprinted warning label or placard, conforming to NEC 702.10(B), warning that disconnecting the EGC to the generator will cause the generator to lose its grounding electrode connection as well, as follows:
Last but not least, you will need to ensure that all connections are torqued to manufacturer specifications with an inch-pound torque wrench or torque screwdriver as per NEC 110.14(D).