I use the same Let’s Encrypt TLS certificate that RouterOS obtains for www-ssl for SSTP and User Manager. The Windows SSTP clients never had any problem with that.
There a a couple of little things that need to be addressed at certificate renewal though. Because I use my domains and not the one from IP → Cloud, I need a script that temporarily opens port 80 (but only on the IPv6 firewall, luckily), enables the www service, request the Let’s Encrypt renewal, then shutdowns www and closes port 80 again. The second issue is that although the renewal process automatically updates the setting of www-ssl with the new certificate, it doesn’t make changes to SSTP server or User Manager and they still have the old certificate. So the script also has to make sure that those settings are updated.