Maybe it's enough to get a USB switch and a Y cable?

I use this model of switch to switch mouse/keyboard between my W7 thin client and daily driver PC:

And I'm planning to get something like this for my W98 PC:

Or maybe these:

I have one laying around for mice, but never tried.

Otherwise I used a D-Link DKVM-2KU to switch between 2 retro/nostalgia PCs. It was cheap, works fine and discontinued.
https://www.dlink.com/en/-/media/consumer_pro … sheet_en_uk.pdf
https://www.dlink.com/-/media/Consumer_Produc … U_QIG_EN_UK.pdf

I went the other way and use PS2 style KVM's as almost all my computers have PS2 ports. Bonus is they are typically cheaper, especially second hand.
although I use a PS2 keyboard, mouse is a USB optical mouse with USB adapter like the 3rd pic in the above post.

For the 2 PC's that don't they are modern enough to support cheap PS2 to USB HID devices like this.
https://www.amazon.com.au/GELRHONR-Adapter-Ke … d/dp/B089QG9Y6H

The USB Switch method sounds interesting, however I don't know exactly how this would handle using my main PC's keyboard and mouse. I could try converting them from USB to PS/2 but my main PC's motherboard lacks PS/2 ports. So it'll be like USB to PS2 to USB practically, and could potentially lead to various amount of issues I think.

The problem could technically apply to using older PS/2 style KVMs. Meanwhile while my finance is in a tight situation, I decided to reorganize my L-shaped desk for the time being. Basically two keyboards and mouses in the front of the desk that could swap places whenever I feel like firing up one of my older machines. It'll do until things are sorted out, then maybe I could look into trying KVMs in case one or few of the devices I ended up buying doesn't work out as expected.

Do keep me updated, because a way to share keyboard+mouse that are both compatible with machines both with USB and PS/2 does seem to be a convenient idea in the long run.