Lightroom Catalog and Image Files Across Multiple Drives?

Wondering about workflow/challenges/etc related to possibly having the image files of the Lightroom catalog across multiple drives.

I have about 140k images from the last ~15 years (could cull some more but that's a lot of effort to trim down). Currently the main files AND the catalog sit on a 4tb WD external USB 3.0 drive. I have a work laptop that is what is typically with me, so I keep everything on the external in case I want to access the files or add images/edit/etc when thats the only computer with me. The 4tb drive is just over half full, so I do have room to grow (it grows faster obviously with larger image files in 2020 than in 2005). The 4tb drive then backs up to my QNAP TS251 NAS via the front USB plug in.

I am contemplating picking up a thunderbolt 3 external SSD to use as the primary catalog/image file drive as I think the current 4tb WD drive is limiting me (Lightroom is pretty slow), but I struggle to justify the cost on a 4tb NVMe thunderbolt 3 drive.

I am wondering what the workflow would look like and/or what challenges I might have if I keep the catalog (i prefer having everything in one catalog although if that is a huge hurdle I'm open to change there as a plan B or C) and my most current files on the fast TB3 external SSD and then keep older less accessed photos on the 4TB drive I have. I think the main thing I see is having two drives that now need to be backed up is an extra step, but wondering if there is any other issue here with splitting the image files across drives and maintaining one catalog. Also any issues with sync'ing to Lightroom web/mobile on this? I have finally got my iPad and pixel running Lightroom mobile and really enjoy that for editing.