rsync cannot have its bandwidth limit parameter changed dynamically. However you could
Split the job into several smaller tasks where each will have a specific bw limit based on the time it starts, eg
rsync a b c d e target:/dir/
rsync --bw-limit=bw1 a target:/dir/
rsync --bw-limit=bw2 e target:/dir/
Use the --time-limit=MINS option
I figured that the only way to solve my issues was to have a daemon running on both sides. Since I didn't want to install Cygwin or WSL on the remove Windows Server (not under my control) I ended up using Unison which works perfectly fine so far. Syncing now takes 10-15 minutes. So use Unison with SSH I downloaded the SSH binary from the PowerShell ...