If aluminum is an option for you, forum members have built bikes with the Gas cycle (use search function)
There are also threads about Seaboard steel frames

I am based in Germany and my AliExpress order on the Ryet Official Store was cancelled after it was initially accepted (reason: "issues with shipping") and refunded

anyone ordered these recently? curious about time until shipping. not sure if LC is struggling to meet demand only on frames or everything.

Aren't the Ullicyc T4 Ultra on AliExpress near identical to the HBR17?
Slight differences in the mounting system to the fork and cable routing ports, but the rest seems very comparable to me...

Incolor, Ullicyc

Might be worth checking with HY Bicycle Store on AliExpress:
https://aliexpress.com/store/1104559635

If you prefer "non-flat / shaped" saddles, did you ever try or look at the Mixed 5D?
If you prefer 3D printed, this Ryet saddle (seems it's called X21 or X6D) is similar in shape:
https://aliexpress.com/item/1005006194842226.html

For reference, I am 1,95m tall with 96cm inseam.
Came to this wonderful forum for my Velobuild VB-R-099 build some years ago.

In regards to your stack height topic: of course this depends on quite some factors, riding preference, torso length, flexibility, etc.
Over the years I refined my bike fit, but would not go below 600 stack today (at least not significantly) while I am targeting around 620 for me these days.

Posted this on other similar threads, already:
https://chinertown.com/index.php/topic,7056.msg106451.html#msg106451
Frames you could look at are Yishun R068-D / Hongfu FM238 / Lightcarbon LCR018-D / Carbonda CFR-505 SL

Sourcing frames and building bikes requires knowledge and experience, especially true for Chinese frames as not everything is always perfect and support comes from far.
Maybe worth considering a European brand such as Canyon or Cube, both have high stack options for tall people.
Or investigating the second hand market?

Do you know if it's possible to buy this new bolt anywhere for users with the old system?

I can not remember how I got my replacement bolt, through the AliExpress seller or from Magene directly.
After I lost the first version preload bolt, I ordered two pieces of the new two piece design, to have one spare just in case.
In the P505 thread there are multiple users that have shared how to get replacement through AliExpress sellers, Magene directly or even through Panda Podium.
Example here
https://chinertown.com/index.php/topic,4569.msg101586.html#msg101586
I am not sure if there are local sources, I guess most people have received replacement from Asia (incl. waiting time).
I remember, that I used an aluminum preload bolt from AliExpress (Shimano style fits) in the meantime while waiting for the Magene part.