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Since my initial question, I did some browsing and googling and thought I'd share my answers here. At least if anyone asks ChatGPT in the future, then there's some real answers...

I have started testing this Lindy DVI-D to HDMI converter, it is about £20 posted new. I figured it ought to be a good option for mid 2000s PCs with DVI and no HDMI, like my dual Athlon MP with a 7800GS (pictured). It caps at 16:9 1920x1080, which I am fine with as I am not sure XP on an ultrawide is a desirable experience. So far it seems to work ok, haven't done any input latency testing, but a quick game of HL1 is good, and the desktop experience is nice and crisp. It also bundles audio, which is extremely useful. The power cable is a bit finicky, however, so I welcome alternatives if anyone has any to post. With that said, I may just make some USB power cables to power off a USB power supply I bought for another KVM-related purpose.

How does this build into the bigger picture, you may ask? Fundamentally, I think I can use an HDMI KVM switch with USB 2.0 to get a stable keyboard and mouse experience across all the computers, and transport audio too. I've found some other analogue-to-digital converters that are powered and will hopefully offer a good artefact-free experience. In principle, I think I will put my keyboard and mouse into the HDMI KVM, use port 1 as a USB pass-through to my DisplayPort KVM, which my modern PCs use, and the rest for my retro adventures.

My Das Pro 6 also works on USB2 via a type C to A adapter.

Next up to test will be these bits:

KVM: 4 Port USB & 4K HDMI 2.0 KVM Switch - NEWLINK | CPC UK

VGA to HDMI adapters: https://cpc.farnell.com/av-link/122-423uk/vga-to-hdmi-adaptor-kit/dp/AV29424 (will test this on a win98/voodoo 2 rig)

The KVM has a remote (important). The VGA-to-HDMI adapter also transmits audio (important).

Hope this post is useful to someone. I would welcome any suggestions, feedback, or improvements!