"Notwithstanding, here's the USB to PS/2 adapter you need.
https://www.ebay.com/itm/252486083292 "

Those pretty much do not work. It may have the correct wiring but the USB drivers must be installed, and if they are then the problem is solved.

I remember building a 98 PC for a buddy of mine. After all the ordering of components, it was off to the local PC store for a few things. They talked him into a USB keyboard. I said "Just how fast can you type ?" but he got talked into it anyway. Problem was he wanted to use a power on password which of course won't work in 98 with a USB keyboard.

The same is true of USB keyboards as well as mice. The wires in the adapter plug are not the whole story.

You should still be able to find an optical PS/2 mouse. I have one. Even though I run XP on that box it works just fine, lucky in light of the fact that I have boot drive select on that machine.