New machine time! Fun!

IMO a 486 is fun but parts are harder to come by and not even the fastest 486 can handle SVGA very well - not even a 166MMX can make Duke3D shine at the higher SVGA resolutions.

None of the games you listed should have speed issues and Voodoo3's have great compatibility so what you have actually sounds pretty good to me - sound would be a problem though without an ISA slot.

Of the machines you listed I'd say the Slot 1 would be the most useful for late DOS games - here's what I use to cover that period if it helps:

Pentium 166MMX tower of power
A Voodoo 3, Vortex 2, Slot 1 based Glide machine

For games supporting general MIDI through a MPU401 interface select MPU401 instead of Sound Blaster for music.

If you had a GFD for your Slot A machine, it would cover a fairly broad speed range for Win98/late DOS game work. I'm actually not sure what the minimum clock would be. If you ever need to look at something slower, socket 7 with a PMMX is excellent due to the wide variety of speeds available and immunity to software like Ultima 7, which is resistant to normal cache tricks.

What games are you having speed issues with? I don't think any of the builds you've listed will be free of them.

Also, wow SB Live!'s OPL3 emulation sounds weird! Descent's music misses like half the notes, and Doom sounds like there is a hanging note bug of some sort (even though it's not MIDI).

Consider super socket 7 with a k6-iii+. You can disable l1 and l2 cache as you go to get a huge range of compatibility with CPU bound games. It should cover anything from a 386 all the way up to 9x games.

486 is great if you have the money and the time to hunt for the parts 😊 .. It took me a year to get mine to a half decent state

Phil plug - https://www.youtube.com/watch?v=fcAqRbFFQPU

I second SS7 platform with some fast AMD cpu (K6-2, 2+, III+). I'd take this over a slot 1 because you may need to slow the system down for some games, and slot 1's slow down too much usually (think slow 386). My personal favorite is a 200 or 233MMX Pentium. These have lots of slowdown options, their only weakness is not quite enough horsepower at the top end for some of the latest DOS games in SVGA. So that's why I suggest an AMD K6. Something around 500Mmhz should make those SVGA resolutions more playable, but still be able to slow down at least into 486 territory.

I agree with clueless, except I'd recommend something with more top end grunt (like your Athlon). Running Duke3d, for example, at high res is very CPU intensive.

If you have a CRT at hand you can use a fast 486(133+) since you have the 320x200 option without blurring. Or use an overclocked P1 or standard P2/K6 for something like 640x400(+fastvid or mtrrlfbe) which is high-res but still has a nice crispy look.
On LCDs requiring 1280/1600 you should have a really beefy CPU at hand... an Athlon seems right. I would opt for the highest clockspeed you can find for a cheap price.

On the subject of late DOS games, Strike Commander and Stonekeep would be two good choices.