It is working almost as it needs to but as I’m not experienced I’m wondering if my configuration is a good way of achieving my goal. Could I improve something ?
Ideally I want to restrict the users ability to change the configurations of Remmina but I didn’t find a good solution yet.
It is a bit slow to boot. Maybe some useless services are being started and I could disable them ? How could I know what services are not needed by Remmina and could be disabled ? Or would it be easier for me to use a minimal distribution like Arch ? Could I still apply my configuration steps or am I going to need to change everything ?
Don't roll your own if you can help it, just use a distribution dedicated for use as a thin client. I was co-incidentally just looking into this last week and came across ThinStation which looks really good. There are other distro's too, search for "linux thin client".
You could set their users up with non-power user access on the Windows host and that should be enough to keep them from accessing OS features or things they shouldn’t.
You should look into distros that do this specifically, they are called kiosks. There might be a way to configure your distro of choice to be a kiosk, it is definitely a solved problem.