With 4 GB of RAM you will be limited, so either XFce or Mint are your best bet (and edit their StartUp pref panel to disable some services – that will save you 200 MB of RAM). I wouldn’t put Gnome on a machine with 4 GB of RAM, it’ll start swapping before long.
Not sure why you say that “ubuntu studio is absolutely not an option”. You don’t give any reason why it’s not an option. Ubuntu studio has special scripts to make things like jack2 work in pipewire correctly, for one. I couldn’t get Presonus StudioOne to get any sound on my Mint installation without that ubuntu studio setup script, for example.
Reaper is nothing like Ableton, it’s its own thing, and you’ll need to get used to it. Ardour is another option, Bitwig, and some others I mention here: https://mastodon.social/@eugenialoli/113358203445896735
LMMS is mostly for electronic/midi music (the UI is like FL Studio’s). For recording, you’ll need to download their .appimage dev-build (they implemented it a few months ago). Still no vst3 support in it though.
And you’ll need to get a supported audio interface, you can’t judge audio quality via BT.
Personally, I’d go for Linux Mint and do these things: 1. Install a theme that pleases you visually, 2. Edit startup sessions to not load useless things (I’ve even turned off bluetooth) 3. Uninstall fwupd (you don’t need that on a mac), 4. Uninstall the evolution-data-server 5. install the ubuntustudio pipewire config script, 6. install the daw you like, and use the pipewire connection kit to make sure you get sound out of it.
On my Mint, I’ve been able to get it down to 700 MB of RAM on a clean boot (out of 1.3 GB by default). That gives some headroom to do better web browsing or media work.
Laptops from over 5 years ago are well supported by Linux (for the most part). However, the very modern laptops have bits and pieces that aren’t supported, from fan profiles, to the new intel webcams etc. They will run Linux, but you might fry them if your fans don’t work properly. So your best bet would be to get either an old one (I got a Macbook Air from 2015 with 8GB RAM, works great), or get a Tuxedo, or a System76 one, or a couple more Linux-specific ones. And it’s not because suddenly Linux does worse job supporting hardware, but it’s because these machines are. getting more complex and they need drivers for every little thing. Back in the day, things were more generic (e.g. the fans) and worked with a single driver.