So the pcloud app has some sort of thumbnail caching, so you can easily browse through many photos?
Roundsync currently does not have vfs-cache, I'm not entirely clear but it supports some kind of caching, but it does not persist when the app is closed: FR: Offline Cache · Issue #114 · newhinton/Round-Sync · GitHub. Offline persistence probably not necessary if caching was good enough. However I could imagine some solution that keeps a thumbnail database. If only I could use my existing 1GB digikam thumnail database 
I found several years ago the creator of the predecessor to Roundsync inquiring about adding some kind of thumbnail support: Quick thumbnails preview on RCX - #8 by x0b
Currently this issue may be preventing rclone (through roundsync) directories being utilized in other apps: Folders cannot be shared/accessed via content provider preview · Issue #184 · newhinton/Round-Sync · GitHub.