Not really with mdadm raid5. But it sounds like you like to live dangerously. You could always go the BTRFS route. Yeah, I know BTRFS Raid56 “will eat your data”, but you said it’s nothing that important anyways. There are some things to keep in mind when running BTRFS in Raid5, e.g. scrub each disk individually, use Raid1c3 for metadata for example.
But basically, BTRFS is one of the only filesystems that allows you to add disks of any size or number, and you can convert the profile on the fly, while in use. So in this case, you could format the new disk with BTRFS as a single disk. Copy over stuff from one of your other disks, then once that disk is empty, add it as a additional device to your existing BTRFS volume. Then do the same with the last disk. Once that is done, you can run a balance convert to convert the single profile into a raid5 data profile.
That being said, there are quite a few caveats to be aware of. Even though it’s improved a lot, BTRFS’s Raid56 implementation is still not recommended for production use. https://lore.kernel.org/linux-btrfs/20200627032414.GX10769@hungrycats.org/
Also, I would STRONGLY recommend against connecting disks via USB. USB HD adapters are notorious for causing all kinds of issues when used in any sort of advanced setup, apart from temporary single disk usage.
Not a place in particular, but if you’re driving, avoid any border crossings during peak holiday seasons. Specifically when you’re crossing from the EU into non-EU countries or crossing from Schengen into non-Schengen area. During peak times you might be waiting at the border for hours.