Not to discourage you but first, yes the Remos are more refined than the Geha etc., I have several in my collection. All 12s need the front ring modified to accept the 12 shell. At the end of this modification the front lugs of the bolt do not engage, the rear lug now does all the work. I've never heard of one letting go.
I have a rifled barrel in 20 gauge to do basically what you are contemplating. Having said all that I believe a 71 / 71-84 action would be a better option. The Remos are very nice shotguns and rather scarce to begin with, every Mauser fan should have one. Should you decide to continue you certainly will have nice bore rifle. --- John ** PM me if you wish **