I only know the Firelight, but I believe the Oslo and Castine have the same construction, scaled smaller. I owned the old Firelight 12, but all of the primary castings and assembly were identical to the F600, just modified for required interior differences. Everything was cemented, with the exception of the lid and inside rear burn plate, which were gasketed for easy removal. Base and sides were all cemented. You’re right, it is likely several years before it needs attention, unless overtired.
Agreed on convective designs, if heating from a basement. My Jotuls could crank out massive heat, by the stone fireplaces into which I had stuffed them just soaked it all up, and spit it outside. That is what had me checking the PE Alderlea, before the Ashford was released.