> how would someone be able to break in via the DSM login?
They probably couldn't. But, at the end of the day, the risk is yours to take. Nothing is 100% secure; it's all about degrees of security vs usability. You seem to have taken a reasonable approach to protecting the web service, so that's a good start. Other things would be to ensure that access is logged, and that failed attempts are delayed between retries (preventing brute forcing to be completed in a reasonable time) - not sure if Synology has that or not.
> how would someone be able to break in via the DSM login?
They probably couldn't. But, at the end of the day, the risk is yours to take. Nothing is 100% secure; it's all about degrees of security vs usability. You seem to have taken a reasonable approach to protecting the web service, so that's a good start. Other things would be to ensure that access is logged, and that failed attempts are delayed between retries (preventing brute forcing to be completed in a reasonable time) - not sure if Synology has that or not.