Ask Carbonda, I think the 505SL has Udh. In another forum someone also reported about a 707 with Udh.

Yes it is UDH, I'm still waiting some parts for a first over 3 coming builds.
All with a GX T-type rear + Force transmission 1x

I'm curious about your opinion and recommendations on Press-Fit vs Thread-Together BB86 bottom bracket shells and especially these two in particular:

• ZTTO Thread Together - https://www.aliexpress.com/item/1005005940645297.html
• Shimano Dura Ace SM-BB92-41B - About 20€ in a local shop

Which one would you pick and why?

The thread together ZTTO BBs I tried were great; however, for that price, don't expect to get quality bearings.
You can replace the bearings with better ones. However, one hard issue, especially in the DUB world, is finding replacement plastic sleeves if you crush them. I'm a tinkerer, so it's fine for me, but if you're not, I would recommend the Shimano one.
If installed correctly with a compound to help it adhere to the frame, it should be reliable for many thousands of kilometers/miles.

Still riding the wheelset of my first build, 9+ years ago, on my second/winter hardtail.
And with Pilar nipples and aero spokes !

I've had a few friends who did break their rim while riding, or were hit by a car, or melt their brake pad into the rim while descending mountains,...
Clearly not product faults.
But LB did always send me a free replacement rim, us paying the shipping.

Their new carbon spokes make stiff and light wheels.
As said many times already in the forum... yes, I'm a fan 

Did you consider the new version of the popular CFR-505 ?
I prefer its thinner tubes compared to others, and with appropriate wheels on it, it can look like a road bike too.
It has a bit more tire clearance, the UDH and T47 BB and lighter than its predecessor.

Okay so now that I’ve done some digging and research, I’ve come upon Switfoto, which sells skinsuits and speed suits with an “Italy MITI” chamois…not really sure where it’s actually from but I trust it’s not trash. Suit has good reviews and aero sleeves, which is what I really want.
https://a.aliexpress.com/_mrZfCpW

Has anyone ever tried sewing the back of a cycling jersey onto a short?
Not as aero as a suit, but easier to have a good top/bottom fit, and keep the jersey in place.

...Carbon 3D saddle...

Curious... Which one ?

May I know your height?
I am 174cm ( 5'9") and use a size M frame with a 60 mm Carbonda handlebar. I feel 1 cm short.

178cm, long legs(89cm inseam) long harms.
Everyone is different.
For a coming build, I suggested the guy to go first with a stem+bar first.
UNO stems are cheap to test. And when he's sure, we'll get him the integrated bar.
Cheaper and more precise than a bike fit.