In this case I'd just fill the inside with resin unless you actually blow through the outside later (you can use it on the outside, but i like thin CA better for that personally). The bark hole in the inside bottom looks fairly porous so i think it'd take up resin pretty well. I'd probably do at least two rounds, an initial smaller pour to seal and then another one (maybe mixed with a filler, and possibly some color) to fill.
Tough question! I've mostly quit trying to do that and just embraced the fact that wood has character hah. It's 100% a judgement call, and depends on how you want to do it IMHO
Some resin and coffee grounds (dried - throw them in the oven for a bit..) would probably look ok as a filler in the hole. Saves on the resin as well. I don't think you'd need more than a couple oz... maybe 4 with waste.
I might CA the bark on the outside even if you resin filled the hole.. at least as you get to the "close to finishing" stages. CA works pretty well for stabilizing bark but doesn't penetrate super deep so it's kind of a "do it at the last minute" thing (doing it in a few stages can help keep it all together as you turn though.. so I guess that's more of an "and also give it some extra at the last bit" but "apply some as you go along to help it all hold together as well"). A really runny resin would also work, but then you're buying two resins... cause you'd want something a tad thicker for the fill. The other advantage of CA on that part is that it dries really fast so you can apply, turn, re-apply as you go. Epoxy takes a while to cure and I've never had it go far enough into the bark I'd entirely trust one pour on the outside... so my desire for quick gratification kicks in here
It's basically just two compounds, a resin and a hardener and you mixed them together well.. then pour... They're mixed either by volume or weight depending on the type. I use some little food disposable food service plastic serving containers from the local restaurant supply (they're handy for all sorts of things.. although acetone will dissolve them it turns out..). I've only really used System 3 (general purpose, fast & medium speed) and West Systems (105 for general purpose and gflex for some wood<>metal stuff) basic resins so can't speak much to the wider array available. Generally it's all pretty simple for simple stuff... it only gets complicated if you're trying to do something complicated
