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.

i used to have a 2013 macbook air from the electronics disposal bin and used it for five years as my daily driver. i don't know how well it would do considering all your audio related constraints; but in your shoes i would do some quick and dirty testing with a live linux distribution on usb drive.

i would go with elive because it comes preinstalled with proprietary software; including things for multimedia, like codecs. you could also do it w any other distro, but why not save yourself the extra steps; since we're only looking for breadcrumbs to follow anyways.

i used to use a headset w a usb audio dongle circa 2002 on red hat linux and the breadcrumbs i found back then led me to discover that getting it to work required the proprietary software that the distros wouldn't carry because of the licensing. so i spent days tearing apart my new installation trying to get it to work with the help of strangers on internet linux forums and ultimately failed; i later succeeded with mandrake linux and ran with that for about 3ish years before switching to debian and later elive.

elive has already figured out all the intricate details necessary to get that software to work and i'm inclined to believe that they made significant improvements over the last 20ish years. you can use the fruits of their labor for a quick and non-invasive test that can be the first breadcrumb that leads you to whatever you end up using for your two machines while ensuring your audio requirements will be met; maybe elive can also help with other proprietary software or maybe it's people on the software that they use that are like yours.

also: you lost me on Focusrite Audio-Interface paragraph so i don't understand how it fits into any of this.

On the DAW, the three are good, I use Ardour cause it's a free software, but I've been told the other two are good, specially for people coming from ableton who want something close. Ardour is really a old-fashioned saw like pro-tools.

Check Librazik it's a distro based on debian made for musicale production.

I'd say you don't need a specific distro for what you want to do : a Debian or Arch with KDE could do the trick, but I would recommend to use a lighter desktop environment like Xfce. You may not like it coming from mac but it will preserve machine resources for your audio work.

Ardour runs pretty much on it's own on any distro, you can still do some conf, I suggest to go to linuxmao.fr the website is mostly in French but have a lot of configuration documentation.

This audio interface will not be an issue as it's plug and play on Linux since a while now.