Well, the 486 should serve you well for most things DOS/Windows 3.x/9x. That's been my experience for over 20 years.
Skimming the thread
1.) The 486 DX2 should be pretty capable of covering anything DOS From the mid-80's on up to maybe 1997-1998 at the latest. If it has a Turbo Switch, it may improve your capabilities just starting out even more. The DX2 was a very popular chip and worked. Doom 1 will run on a 486, and so will Doom 2 or Ultimate Doom. I have the DOOM collection on my DX4-100 next to me, and it was running that great as a DX2-66 over a year or so ago. Typically where I find the 486 starts to falter is just very slight on Diablo if you are running something older 486 or with a slower chipset, or the original Postal. Your main problem will be getting older titles to run slow enough conveniently. Some people do things like disable caches, add wait states, or use slowdown programs like moslo on their machines. I find though, there's not much on a 486 that runs too fast to be fun unless it's some old early-mid 1980's DOS Title designed to run on an 8088 and has no speed throttling. About the only title that runs too fast on a DX2 from near it's time period is Test Drive III (which for some reason is supposed to run like crap, but IF ind it quite fun on a DX2 or faster.....reminds me of some 70's movie I saw lat at night with 2 itailian dude driving a Ferrarri ungodly fast by virtue of sped up camera footage, 🤣).
The Amiga 4000, I'm not an Amiga person - well, not yet at least - was actually a whole other animal. The Amiga systems originally had their own O/S,platform, archetecture, totally different animal from the PC. Sounds like you have some kind of PC emulator card in there. While a lot of DOS Software SHOULD work in that kind of scenario, there is always the odd straggler here and there that does not. And driver support for any specific features can be tricky because it's not a genuine PC, but rather an interposter built into an Amiga 4000. Of course, I could be totally wrong and it's just a Commodore branded 386....not sure, someone else on Amiga can chime in, if they have not already.
If it's just a branded 386 though, that system starts to peter out in performance on the later Ultima games (Ultima 7 and 8, from the early 90's), DOOM does not run as well on them, but they are a great center-point for DOS Only stuff (whereas the 486 is a great center-point for everything, with the right tricks a 486 will even boot Windows XP....nuts as it sounds). But I don't want to give any absolutes on the subject because I've never used one of those other platforms with a PC emulator card in it.
3.) The Acer Travelmate could be a good choice for the more complex games, but if you are really into saving space and wanting a good all-arounder that won't give you much issue It might be better to cover the whole gamut. Also, being a PII, it's just new enough to do some modern things well, but just old enough to still talk to all the old stuff, making it a excellent Tweener (assuming it has a CD drive and you are willing to seek out the apropriate cards for a network connection).
And that's my .02 after skimming the thread a bit, sorry if I repeated anything but this is my perspective after doing retro-computing with PC's since the early 2000's.