Hi tophee,
Not an expert in pcloud nor serve sftp, but I would first try to make a SIMPLE and SLOW setup where there are no issues (except performance) and then gradually increase/double one parameter at the time from there until you start to see errors or many low level retries.
I would also simplify the command initially and test on a testfolder with a few files.
In your situation I might start with something like this:
rclone copy pcloud:testfolder ./testfolder --ignore-times --checkers=1 --transfers=1 --tpslimit=0.25 -vv
And then gradually move towards:
rclone serve sftp pcloud:Backup/ --transfers=X --tpslimit=Y ...