Yes indeed! You can't switch databases once this has been setup from Forge. However installing MySQL yourself shouldn't be a hard task and you can then simply use that, you can't manage it from the interface but it should work.
You can follow this tutorial to setup MySQL on your VPS: https://www.digitalocean.com/community/tutorials/how-to-install-mysql-on-ubuntu-18-04
Note: This is for ubuntu 18, there are also similar articles for ubuntu 16 and so on!