RetroGaming

If you want to emulate those systems, then yes, you're going to need a fairly beefy computer. You could, as others have suggested, buy a good secondhand system and upgrade it with a GPU and more/better RAM.

But I want to pass on a warning as someone who also loves emulation and wishes they could "have everything in one place": a lot of emulators just aren't there yet, but some people are eager to kid themselves and others that they are.

16-bit systems and before typically have outstanding emulators available. Some systems from the next couple of generations are also very reliable (e.g. PS1, Dreamcast), while others mostly work well with minimal tinkering and only a small handful of exceptions (e.g. N64, Saturn). But after those, the reliability of emulators drops off fairly smoothly. Even the venerable PCSX2, for example, will run almost every known PS2 game in some fashion, but many games outside the biggest hits still have problems that make them terrible. And I don't mean picky things like, "Three notes in the bassline on this background music are slightly off," I mean, "The walls aren't rendered in most areas."

I really recommend having a good look at the compatibility lists for emulators you're interested in before you dive too deep down this hole. It's one thing to have a powerful PC already and think, "why not give it a go?" but another thing to build a new (to you) PC specifically for emulating these systems. I suspect that you may have been spoiled a bit by that fact that even the RP4 only has enough power to run those more stable emulators for older systems.