5 years ago, I've built a ~1100g wheelset: ultralight 29er hookless rims + Novatec 411/412 hubs + sapim spokes. And still riding it.

https://chinertown.com/index.php/topic,3144.msg25520.html#msg25520

Did anyone who built up one of these feel the need to use foam tubing to reduce cables rattling within the frame?

You should do it every time.
One day or another a hose will rattle.

If you use foam tubes, be sure to take the right diameter or it will be painful to put on.
Another way is to tighten 4-5 short zip ties every 10cm by the BB holes when pushing the hose.
Mark the approximate start and end, while still outside the frame so you know where to place them.

For 28mm tires, I use 30ml of Orange regular sealant.

Depending on the season, add back 30ml after 3 or 4 months.
Checking with a 2 hex screwdriver, through the valve placed at the bottom.

Then my tires are in general worn out before a 3rd refill.
Coupled with Dynaplugs, it has been years without an unfixable flat.

...but I'd go 909 for the extra bottle mount.

If for some reasons, you prefer the look of the 936, there is a double bottle mount that works well: https://www.aliexpress.com/item/32975689555.html
Put some thread locker on the screws, and no interference with the legs.
At least you can try for the price.

Yesterday, I noticed the drive side BB bearings seized 
While removing the BB, some accumulated water went out.
I drilled a 2mm hole at the lowest point of the frame, to let the water flow out, hopefully to make the bearings last longer.

...After riding in a few years in spexcel jerseys where the zippers started to fail...

They changed the zipper, and have a better one now.
I found it less appealing at first as it is more visible, but it is sturdier than before.

...My only complaint is that their packaging is too fancy. They send the bolts in a metal box which is padded with form fitted foam. In my opinion it is just too much waste. I would have been perfectly happy just receiving the bolts in paper bags.

Did you tell them ?
I didn't like it either, and complained.
If we are many they may improve this, and save some money too.

...Regarding the versions with exposed pistons, it seems like that was actually the 2nd generation design done by a company called IIIPRO who produced them until Onirii took over the project.

I'm impressed. How do you find that much details about it ?