Ahoy! I figured I've got enough information now to warrant making a thread in case this helps others in the future.

I recently picked up this Softek 5HXA with 16MB of EDO RAM and 5K86-PR75 CPU (SSA/5 model, but has both brandings on it) for a song.

Motherboard Info:

• Chipset is 430HX with PIIX3 southbridge. While the chipset supports USB 1.1, the headers and accompanying SMD components are missing on my board (bummer) but seem like they could be added as it *DOES* recognize the controller in the BIOS.
• PS/2 mouse header.
• 256kiB pipelined burst cache on board, with COASt socket for additional cache. Tag chip mine arrived with is a W24129AK-15 (16k x 8, 15 ns). Socket is 28 pin for tag chip but the 2x UM61L3232AF-7 (32k x32) are QFP.
• There's a 'hard-bridged' jumper on the board with labels for P54C and P55C that is soldered to P54C. Right next to it is an unpopulated area that looks like it would get filled with VRM components for the split-plane P55C voltages. Not sure what you'd use for BIOS if you tried to add the missing components, though.

Thanks to mkarcher with this reply in the "What retro hardware did you buy today?" thread for filling in some of the above info.

mkarcher wrote on 2020-09-22, 21:37:

The Softek 5HXA is actually a rebadged Soyo SY-5TF mainboard, you should be able to find info on that board. I happen to have the original board at hand. The onboard tag chip is just 8 bits wide, so only 64MB cachable area. There are no more than 8 data bits on a DIP28 cache module, so no way to get to 11 data bits on the tag RAM to obtain the whole 512MB cachable area. And the worst news about that board: I traced all the pins on the coast socket, only to find out that the coast socket also has just the low 8 tag RAM bits connected. The remaining three tag RAM bits seem to not be fed out from the 82439HX south bridge. Thanks to the BGA case, you also can't simply add bodge wires to the three high tag RAM bits, so tough luck on that.

The south brige is indeed the PIIX3.

BIOS Info:
Board arrived to me with an Award BIOS chip showing copyright 1995;. Awdflash shows it as being a MX28F1000AP /12V meaning you need to use version 5.2B or higher to flash (I used 5.4).
Clock chip is a socketed ODIN 12C887A (dead, of course). BIOS String was 2A59FS29 and had specific Softek customizations in it.

Board can be flashed with patched BIOS (string 2A59FS2AC) from Wim's to gain access to USB controller (it just doesn't even show up with Softek BIOS and no option to enable) and 128GB HDD limit; possibly other options but haven't fully documented.

Putting files I used here for others.