phonetrack seems to be hard to replace
Have a look at Traccar for a replacement.
In my experience file browser is much nicer than nextcloud, but I don't currently run either since I don't need a web based file manager. I have SFTPGo providing WebDAV for my phone to use, and Syncthing to keep my laptop, desktop, and server in sync.
Have you benchmark the disk locally directly on the proxmox host? Need to figure out if this is an IO limitation, CPU limitation, or something else.