I've not tried these myself but I believe the following may support scaling:
- FreeRDP - Remmina seems to be part of this project now but they also have their own client (freerdp-x11 in the Ubuntu repo), is this different to Remmina?
- Grdc - Correction, this is now Remmina of course which you have already tried.
- If you can stand some KDE magic, I think that KRDC may also support scaling.