You are probably wearing those couple of cogs out.

Same as when on the road. .5% for 11 speed and .75% for 10 speed Shimano. You are right it should last longer because of the lack of road grime. I would still wipe it down though and lube it on a reasonable interval.

Spend $10 and buy yourself the Park Tool CC-3.2 -- If you replace your chain on time I can't imagine ever needing to change your cassette or chainrings.

My 2 cents, YMMV

I had a dedicated trainer bike for 12 years - 8 speed with Shimano Sora. I never replaced the chain and cassette in those 12 years. Both still looked totally fine despite heavy use. I stopped using the bike because the derailleurs and bottom bracket were rusted out and had very high friction and I didn't see any value in replacing the components.

The bike I bought new in Europe in a department store, it cost 300 euro at the time (not on sale). Ugly and heavy bit it fit great. Perfect trainer bike.

Thought id bring this back up.
Im finding my direct drive turbo (lemond) feels a bit like pedalling through mud in certain gears. (middle 3-4) where I spend most of my time. The cassette isn't old at all, but looking at the teeth those 3-4 have a more asymmetrical trough, with a deeper cut in at the front.

Would this be the reason they feel like this?