That most likely means you have another service starting early, writing files to your mounted location, which hits the empty issue so you'd have to identify what's writing there.
Unknown as you've only shared logs that show a problem. I can't see your screen or what you type so you share logs and detail what you've tested.
Well, sure. You can't umount a file system that's in use on Linux. If you try to, it'll never unmount. You have to stop all IO/processes on the mount first before you want to unmount it.
Utilities like lsof as root can you what is running against a mount and fuser as well.
Those lines make things worse. The way systemd works is if you can't stop a service after a period of time, it kills the service which leaves the mount in a non working state.
For mounts, you have to stop all IO/processes and then unmount.
When you boot, you want to make sure your dependencies are configured properly so they don't start until after your mount is there if you are writing data to your rclone mounts.