Onirii make some 200~210 ones, theyre good so far, only the grease inside them is quite thick so they don't spin back into neutral position very quick

Pretty sure those are SPD SL (road), not SPD (mountain).

We'll go M19 is a claimed 300g but I haven't seen anything on AliExpress substantially lighter than that. I think Xpedo have some lightweight options but they're pretty pricey.

Mainly that the default PES P505 base setup is a decent, cheap 24mm steel-spindled crank. At the prices I see on AliExpress, it's about the same price to buy the P505 base and the P505 on ZRace cranks, but the ZRace cranks seem to have a pretty poor reputation so you'd have to factor in buying a different crank. While the spider not being compatible with anything else is a downside and the crank is not particularly light (slightly heavier than standard 105 with 105 rings, I forget the specific number but maybe 840g?), it has been a set and forget experience for me after install.

If you don't mind an aluminium 24mm axle, I agree that Riro crank + P505 is the best budget lightweight option. If going for a DUB crank, the Lexon R1 DUB + XCadey Easton spider is about $60 USD more expensive arguably a safer option from an axle perspective. The other great option as well is a second-hand D1 Red crankset if you're lucky enough to find one in the length you want, and then using either an XCadey or Magene spider for 110BCD rings.

I'm confused. How do those round rubber things work?

My impression at least - and I could be wrong - is that the intention is to slide them onto the bars and up to the tops in the initial build (before putting shifters on) so you can hide the cut piece at the top end of the tape (rather than using electrical tape).

I could be entirely wrong though!

It's an alternative to the finishing tape to make for a no-tape look on the tops. I don't use them.

You've listed pretty much every type of bike there - endurance, lightweight, aero.

What are your priorities in a frame? Price? Weight? Manufacturer/retailer support? Handlebar options? Custom paint? Upright geometry? Aero features?

Are there any bikes from big brands that you like the look of? Hate the look of?

What parts would you be intending to build it with?

It's a shame they did not learn from Campagnolo's mistakes on how to optimise lever ergonomics for a single shift down. This is very much like Campagnolo's early Powershift levers (the 2011-2014) which are pretty generally disliked for the same reason - the lever is in the same position as the Ultrashift levers but can only do the first shift to a harder gear (which L-Twoo seem to have done here). When they revised it in 2015 to be more like the EPS thumb lever (which sits much lower and is angled in) it significantly improved the ergonomics (I'd even say they're one of my favourite mechanical shift levers of all time).

That's crazy cheap - I wish I'd known about before I started my recent gravel build!

CWind is the same as SPCycle (or at least they share a lot of models - assume it's two different fronts for the same factory).

Plenty of threads on SPCycle frames here - the one you've linked is the same as the SPCycle G058.