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):
(C) Multiple Circuits. Where a single equipment grounding
conductor is run with multiple circuits in the same raceway,
cable, or cable tray, it shall be sized for the largest overcurrent
device protecting conductors in the raceway, cable, or cable
tray. Equipment grounding conductors installed in cable trays
shall meet the minimum requirements of 392.10(B)(1)(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.
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.
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:
FOR CONNECTION OF A SEPARATELY DERIVED
(BONDED NEUTRAL) SYSTEM ONLY
FOR CONNECTION OF A NONSEPARATELY DERIVED
(FLOATING NEUTRAL) SYSTEM ONLY
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:
SHOCK HAZARD EXISTS IF GROUNDING
ELECTRODE CONDUCTOR OR BONDING JUMPER
CONNECTION IN THIS EQUIPMENT IS REMOVED
WHILE ALTERNATE SOURCE(S) IS ENERGIZED.
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).