Install leaving old 'Home' intact

TL;DR: as long as you leave the ext4 alone there will be no sorrow.

First a little context.
Garuda uses btrfs, which has the concept of subvolumes (half way between a directory and a partition, so to speak). One of those will be @home (the new one made by the installer, not your existing one).

So, just make sure you have enough space (either free space or an existing partition with nothing you care about, e.g. the old root partition with Mint) and install Garuda there.
Of course, double check in the installer that you are not touching the existing home in any way.
Now, technically, the one in ext4 isn't a home anymore, but everything is still there.
You can simply mount it and access your files. Garuda's partition will be btrfs, your data still ext4.

Or, you could merge one into the other (both ways can make sense, choose what is best for you).
If you'd like your home to be the ext4 partition, after copying over the dotfiles (making sure not to clobber anything you'd like to preserve) switch your home in /etc/fstab (that is, comment out the line with /home btrfs subvol=/@home, and mount the ext4 as /home).

In my case, I had the very same situation and it worked without problems.
A backup is always advisable though.

edit: last but not least, welcome aboard.