Music app on Monterey not opening very large iTunes library from Mojave

I have recently upgraded to a new Macbook Pro running Monterey 12.5. I have been apprehensive about switching from iTunes on Mojave to the new Music app with such a large library on Monterey and as expected I am having issues.

My library is very large, ~365GB / 40,000 items, mostly .mp3 with the odd .wav. I do not use Apple Music. I would really like to retain 'date added' metadata as that is important to me.

I keep my iTunes library on an external drive previously as my local was too small. However, on my new MBP there is enough space to store it locally and I am familiar with choosing different iTunes libraries when opening iTunes previously.

I have:

• copied my old iTunes library folder from my external HDD to a folder on my new Macbook Pro.
• this folder is located inside my Dropbox folder
• opened the 'Music' app while holding down 'option' to choose the library
• chosen the 'iTunes Library.itl' file on my local
• Named the new 'Music' library and saved it in the same folder as the 'iTunes Library.itl' file

Once I have done all of this the Music app shows with a loading spinner in place of the song list, and I left it overnight to process everything as I thought it might take a while. But it was still there 8 hours later with no visible change so I assume it either was taking an extremely long time or did not work.

What is the issue? Do I have to wait longer? If so how much longer? Do I need to move it to the typical location within Documents/Music for the 'Music' app to read the old iTunes library properly? Is it not going to work inside a Dropbox folder, I see no reason why it shouldn't? Do I need to try another approach?

I am confident cracking open Terminal and copy/paste some commands if I need to.

Thank you!