Sync issues making Lightroom Classic and ecosystem unusable

Hi, I'm on:
Lightroom Classic 13.1
macOS Sonoma 14.3.1
Lightroom Mobile on iPad and iPhone

I use Lightroom Classic as the source for all my images, so they're worked on and travel only in one direction, out to my iPad and iPhone in my art studio for sketching and painting, except for a period where I was uploading to Lightroom Mobile albums on the fly from my iPad camera roll without realising what I was doing. The only other problem I had was when a software update on the devices decided to upload everything from my camera rolls without asking. I think that's all solved now but both those things caused a lot of problems.

I have two locations for images:
1. An external hard drive with photos from my cameras, these are images I take for my studio work.
2. A native folder 'Collected Images' in Pictures, containing a massive folder structure of found research images. This structure is synchronised into Lightroom Classic, and set up as albums in the same folder structure synced to my devices.

'Collected Images' is really the engine room of my Lightroom use which I use daily for ongoing research, currently there's about 135 subject area folders inside it, with sometimes up to 100 subfolders inside each of them depending. I read somewhere there is a maximum limit to the number of albums you can create but I don't think this is anywhere near that.
My external hard drive images have their own folder and collections system, which is then replicated in Collection Sets and Collections synced as albums, and moved into the replicated system on my devices. So a lot of Mobile albums, but I don't think I'm anywhere near reaching the limit so I've ruled that out as a problem?

- My catalog lives in its original location in Pictures/Lightroom on my desktop.
- Currently there are 130320 synced images, when I reslove this I'll probably do a cull

I've had sync issues over the last six months, the previous one was similar to my current problem. The previous one (during 12.5) had a corruption creap into the catalog somehow, I never received any message that it was corrupted, I optimised it, but after trying multiple rebuild sync data sessions which would always end in crashing my entire system, I tried an older backup. This seemed to get it back working, and I just redid all the work to catch up, no problem. I think this was caused by me absent-mindedly disconnecting my hard drive early during a back up.
The latest issue was when freshly made albums were showing up empty on my devices, so some of the communication was working, but they would never fill up. I gave them several days, then decided to do a rebuild sync data which seemed to put the images into the albums, but then all of sudden Lightroom Classic was trying to sync a massive amount of images. When I try to rebuild sync data again and again it always ends with the entire system running out of application memory, and often with Lightroom Classic crashing. Image count goes up and down wildly, which is to be expected, but it's just not resolving, so the whole ecosystem is unusable now. Everytime I start Lightroom up now it heads towards crashing and locking up my entire desktop. I'm making sure the rebuild happens with my external hard drive connected.
Curiously, I tried a number of older backups this time, but it just seems to pick up with a similar amount of images it was trying to sync from where I left off with the most current catalog. When there's info in the sync preferences panel (not always), it always seems to be around Metadata. 
I really want to avoid Delete all Synced Data, because re-syncing all those albums and moving them individually into the folder structure on the devices is incredibly tedious and time consuming, but I realise I may have no choice! I do this on the iPhone because it's faster, but it can be up to a minute per folder, Lightroom Web is not much better.

Looking for some help thank you!