I did it!
I managed to complete my first (ever, in almost 20 years of on-and-off MMO playing) tanking of content!
Sunken Temple of Qarn. Two sprout DPS who did their best, and a very patient non-sprout Healer who explained a few things and treated me with great care.
Died twice (Teratotaur, 'doom' mechanic; couldn't parse if it was a tank-ignore or a tank-do mechanic in the fray), but didn't wipe! Still managed to clear each boss and pull in first attempts. Still a bit more mobile than I should be, but by the last boss, I was keeping the boss locked off while contributing elsewhere where I could as well as my DPS.
Even managed a couple of good double pulls (I've always known the strength of Arm's Length, as someone that loves solo content!~)
And would you look at that - one single loop of the daily leveling roueltte and I jumped two whole levels! Another swing like that and I'll be well clear of the plateau upon which I was stuck...
But...
Maybe... I could enjoy this? It's a lot faster than squad grinding (at least, the leveling roulette is...), but... It is a lot more pressure than I'm used to, but fight knowledge would absolutely crush that. So really, the answer is... More... Roulette!