The width of the cogs between 11 and 12 speed is very slightly different, and if you've making custom ratios the ramping will likely be out. On the 12 speed cassettes I have at least one of the cogs is dished slightly and has a thinner spacer than the rest.
You might change some cogs and get it to work - e.g if you had a worn / damaged 14 and changed it for an 11 speed cassette 14 (where the next cog was a 13 /15 in both cases) - but if you're attempting to make custom ratios and changing significant numbers of cogs you either won't get it them all on the freehub and the lockring to engage or the shifting will be atrocious or both. .

I'm very much liking the looks of the new Riro cranks, as I want to move to 165mm cranks on my Ultegra 8100 equipped race bike. But I want to use the original rings. I'm struggling to find someone that sells just the Shimano interface to 4 bolt 110 BCD spider. Anyone have any ideas? Would I just be better to get Lexon where they come out of the box like that?

I don't like the alu / steel mix cassettes but with only the largest cog as aluminium... it could be OK. On the 12 speed cassettes the lowest gear is generally so low it doesn't get that much use, and you'll always be shifting from or to a steel cog so when it does wear, it shouldn't completely kill the shifting.

However what annoys me is that the headline price is just for 11-30, the 11-36 I want for my Marmotte set up is a full £20 more which pushes it beyond the price of all steel monoblocks (and TBH I value the all steel reliability / longevity over the tiny drop in grams). I will likely bite if it drops a bit in price.

Not hugely - it will still work and it will be hidden from view. But it would bother me - I usually get a sheet sander out after I've cut my steerer tube and even it all out which would fix that really quickly (and if you don't own one, they are a really useful DIY tool and in the UK cost from about £10, so not much). Make sure you do it outside and wear a mask as you would when cutting it.

I've got 2 sets of the Elite Ent Gravel wheels and they are ridiculously good for the money. They are around the 1600g mark for 38mm so not super light, but bang up to date with 25mm internal width and 32.5mm external. Look very smart with subtle black decals. I haven't given either of my sets proper abuse to comment on reliability but certainly seem very well made, and many on here have had them or earlier versions for a while and speak highly of them.

TPU tubes are pretty cheap - I love my RideNow TPUs but there are many other brands now and not much between them. IMO not worth paying for the ultra light, as the bog standard are already about a third of the weight of butyl and most importantly pack way smaller. But whatever you can get a decent deal on. Buy 4 as one of the main advantages are as spares, and you wouldn't want to find out the foibles of them on the side of the road fixing a puncture.

If I wanted a genuine Shimano groupset I wouldn't be going to AliX; there are a lot of fakes and IMO the discounts don't make it compelling over what I can get in the UK at places like Merlin anyway. But I would substitute out Shimano parts with brands you can get on AliX for things like chainsets, chains, rotors, cassettes if I wanted to build a nice light bike for a budget; or even going the whole hog and getting LTwoo or Sensah.