You can stop the Syncing by unlinking the Cloud Storage and keeping the database purely local. Just relink when you want to use it on another device or desktop.
As far as large libraries are concerned, from my experience I think that you'll encounter problems.
I have a database with 7 libraries, the largest of which is just shy of 50,000 records but others around the 30,000 mark which I use on a Galaxy ZFold 6 and a Galaxy Tab S11. The ZFold struggles with the addition of new records in particular, meaning that I often have to close the database and start again. The Tab S11 does this far less often. I have previously been advised by the developer that any library over 50,000 records will struggle, but they are looking at better handling of large libraries. Timescale unknown.
Don't even think about trying to work in 'Table' view with libraries of this size.
I persevere because, like you, I think that Memento is miles ahead of any other portable solution I have tried
By the way, I find the desktop version pretty well unusable. It's just too slow.
Simon