any effort with Bootstrap protocol ends up sooner or later in full CPU usage and reboot is needed.
How long does it take to get into that state?
In a minute or two. Once I saw a looping message in a log, something about trouble parsing bootstrap messages. I can try to reproduce that If someone is interested.
Why do you need automatic RP discovery? If there's only one path to the RP, and its IP rarely changes, if ever, static RP is not only perfectly fine, it's faster and more reliable. Protocols like BSR and AutoRP are necessary only when your network topology keeps changing, as when you've got a multipath situation and you can never predict which leg will be available at any given moment.
Exactly that is the reason. For a case the network splits in two or more parts. Namelly, our case involves not only one source but every receiver is also a source of multicast (the amateur radio DMR repeater network). If the network splits, separate parts should continue to work separately and join when network comes back together. This is what Bootstrap protocol is there for and I'd like to use it with MikroTiks in our DMR network, where every router will be both a BSR and RP candidate.