The thing you want should be stable, but underperforming.
Let's start with forgetting the beloved IBM PS/2 30-286, which crawls by nature.
You really should get yourself any 386SX-16 board. Without cache of course. Pair it with 512kB ram (2x256kB).
A 486SX-25 with ISA only would be ok too. Consider them both a sloth on holiday.
Your RTVGA really matches it's speed: a snail on valium. Still pretty compatible with early VGA and EGA stuff, ok support for Hercules and CGA if I recall.
Same goes for C&T 451, Oak OTI030 and Trident 8800. Bottom line would be the original IBM VGA controller, denying quite some later standards. Looking at you, model 30-286!
Just about any generic multi I/o controller may do. Getting a separate serial, parallel, fdd and HDD controller makes it cumbersome.
Assuming you won't set for an ESDI or mfm hard drive, you really should look out for a Miniscribe 8450AT 40MB stepper drive. Or any Connor CP304x 40MB drive, but that's already quite a bit faster (still not fast). Both turtles on weed, matching the snail and sloth above.
Get yourself a 720kB floppy drive. Seriously, this bottlenecks the fun. Want to use a floppy? No 1.44 here, get fiddling with other computers too get your data across on a modified set of floppy disks.
For sound, your card may be a less than ideal option, but you may consider the Avance Logic ALS100+ (yeah, I said it: the plus version which is really a minus). Yay, a 16 bit sound card! Oh wait, it's high address is mostly incompatible. While the opl2 is a cool thing on an XT, it would be a mismatch here as well.
Of course you'll need a bus mouse controller. Yeah, it should work ok but you'll get irritated as soon as you're trying to plug in a PS/2 mouse one again.
Last but not least: get a high pitch black& white screen. If you didn't get a headache already, this assures it.
Next, get the depreciated DOS 4. I never used it, but it's said to be buggy. Versions 3.3x and 5.xx would be more appropriate if you actually want to get the thing better. And windows 3.0, to fill up half the hard drive while it actually is useless: lots of programs want 3.1x and the system is too slow any way.