I'd put Slackware on it and run XFCE, Fluxbox or add Openbox from slackbuilds.org. Slackware is in beta testing for 15.0 at the moment, but you might be better off installing 14.2 anyway on a machine that old. If you're OK with the older install, you might want to try Salix. It's got a nice XFCE desktop configured for you, comes with a gui package manager, etc. Another good Slackware derivative that's based on Slackware -current is Zenwalk. It's compatible with Slackware and has a very nice XFCE setup. Really, though, I'd go with Openbox and Tint2.
If you like BSD the best thing to try would be Void, which has a desktop child distro now, called Project Trident. If you want a fully ready-to-go version of Slackware based on -current and running XFCE (with others available), there's Zenwalk. I haven't tried Project Trident , but I've had Zenwalk and Void running very smoothly on my 4th Gen i5 in the last 6 months. Do you have an SSD in that box? That's the best upgrade you can get if you're still running a hard drive.
Second Void. If OP wants to try something different, I'd recommend Zenwalk. It's based on Slackware -current, so it runs SysV init, but has its own package manager and some other things to make life a bit easier than standard Slackware. The default desktop is XFCE, but it had a nice KDE implementation last time I tried it. It's a bit of an out-of-the-way distro, but since it's Slackware-based, there is a ton of third-party support out there. As much as I like both, I think OP might be a bit too picky for either.
This makes me think of Zenwalk GNU Linux. You can run a LiveCD and then shut down after you're done. I use to use it at the library to get the broken computer in the corner connected to the library catalog. This saved me from having to go downstairs for lookups.
Give Zenwalk a try. Don't be put off by its Slackware roots, it's a fast, tidy system that has a nice package manager with dependency checking, etc.
Personally, I think you need to install something with XFCE, then install Openbox and a panel like tint2 or fbpanel. Properly set up, Openbox with a decent panel is just as usable as XFCE. You can start any XFCE services you need to run inside Openbox by putting them in the Openbox autostart file. You can use Compton/picom as a compositor, but with that machine you might not want to.
There are very good Openbox distros, but if you like XFCE, then you can have the same XFCE apps and services you're used to, but inside a lighter environment.
Have you ever heard of zenwalk? I haven't used it in a few years, but for about a decade it was my goto distro. It's based on Slackware, But a bit easier to use, and it targets older hardware (not too old though, they dropped support for 32 bit processors last year).
Zenwalk comes with it if you want it, and if XFCE is good enough for Linus these days...
Personally, I'm a Fluxbox user most of the time, but it seems useless to try to talk people into that until they've been on Linux a while. Too bad.