iphone 12 – Photos and iCloud library issues – lost albums

I have 30k photos on my iMac (mid-2011, High Sierra) going back to the 1990s, sorted into albums which is the work of many years. Although it’s backed up (Time Machine, Backblaze), I thought it would be very useful to use the iCloud library function to be able to access all photos on my iPhone (12 pro) and iPad (Pro, 10.5 in) instead of manually managing syncing of albums via iTunes – usually to show family and friends holidays and recent events. Have also recently bought a MacBook Air, and would like to put the photos on there too, with an eye to eventually retiring my 9 year old iMac.

Took 4 weeks of overnight uploading to get the photos to iCloud, and then turned on iCloud sharing on my phone – seemed to work ok. Then turned on the iCloud sharing on my iPad and appear to have lost half my albums and a few photos. The photos I’ve managed to find on my iPhone deleted album, and recovered. The lost albums are now not showing on the iMac, iPhone or iPad, nor online if I log onto my iCloud account and look at my Photos there. My questions are:

1. What’s the best way of restoring the albums – presumably there is a file somewhere I can recover from TimeMachine on my iMac?
2. Has anyone else had similar issues – is iCloud photo library robust enough to cope with multiple devices, some of which are not brand new and running the latest software?
3. Is it just teething issues, or if I continue to use iCloud photo library am I looking at random problems like this in the future?

I’m going to be really disappointed if this doesn’t work properly – I’m just not prepared to lose years worth of work and potentially precious photographs, and will turn it off it it looks like being unreliable.

Thanks in advance for any help or comments!