My bad, I realized my comment reads a lot differently than what I was trying to say. Linux mints release schedule is not bound to Ubuntu. Linux mint gets a new major version every two years (although this is not strictly set) while LMDE usually gets a new major update with the new Debian version, but because Debian has been around for a lot longer than LMDE the number is higher.

Contrary to most advice, if you find something that’s compatible with a Wayland session (basically Gnome or Plasma) you might be pleasantly surprised.

I found that to be by far the closest I got to a macOS-like experience with Linux on a retina Mac, in terms of fluidity, trackpad scrolling and responsiveness.

this is a notebook with an Intel Core i5-4278U @ 2.60GHz (2 cores, 4 threads) with 8 GB RAM and installing and upgrading on xubuntu 23.10 was already really, painfully slow.

Have you put an SSD in there, or are you still running on spinning rust? In my experience, even a cheap SSD will make a huge difference.

Several posters have argued that LMDE, like debian, is barebones, whereas LM is ideal for an end user with not much idea about linux

I believe I'm the only one in the previous post that used the term. But, I believe a misunderstanding has occurred. Debian, plain old Debian, is (relatively) bare-bones. And with this, I mean that extra tooling and what not is absent. Sure, these extra tooling etc come at the cost of what some might regard as bloat. But, ultimately, its absence should not affect performance in any significant way (so not positively, nor negatively). Thus, LMDE and Linux Mint are actually pretty close to one another. LMDE is basically just Linux Mint (Cinnamon edition) but with a Debian base instead of being based on Ubuntu.

I also want to future proof it as much as possible, which would mean using the OS/DE that uses less resources.

Excellent OP. Thank you for providing this insight on what's important for you. With this information we'll be able to offer better help. So, as you've excellently noticed already, Xfce is pretty good if you want a very functional machine that doesn't suck a lot of resources. So, I totally support your decision for Xfce over Cinnamon as Xfce is simply less resource intensive. However, 8 GB of RAM should be pretty fine. Even GNOME should run wonderfully on 8 GB of RAM, so Cinnamon should not cause any troubles. But, if you've still got concerns and if you're already on an SSD, then continue using Xfce as it's otherwise one of the better DEs out there. But, if you're not on an SSD yet, then consider slipping one inside; it will matter a lot.

Regarding your actual query, installing Xfce in retrospect to LMDE should work, but you might get yourself into more trouble than it's worth. Therefore, I'd advice you to simply get Linux Mint Xfce Edition and call it a day. Going for the Edge ISO (which by default comes with Cinnamon) for the latest (and greatest) kernel and retroactively trying to setup Xfce should (once again) cause you more troubles than it's worth it. So, in the end, I'd like to recommend you either Linux Mint Xfce Edition or MX Linux (which is based on Debian Stable (so not Ubuntu) and actually defaults to Xfce). Honestly, they're mostly two flavors/interpretations that try to accomplish very similar goals. So, you should be fine with either one of the two.

That's weird, I have a laptop that probably even weaker (Pentium 2020M with 4 GB of RAM) that used to run Ubuntu fairly ok until about 4 years ago (but it has a SSD).

It's now running Manjaro pretty well, just can't compile some Rust-based apps because it runs out of RAM. 😄 But I get them from Flatpak instead.

If Ubuntu went off the rails during these last few years it's a pity. Anyway, it should not be indicative of how well Linux runs on such a machine, just try another distro.