D&D 5E Final Boss (tm) for a level 11 campaign.

[J-H]

And with the spells and a few HP more (let's say 30-50 bonus HP, a very good roll with the 19d12 ^^) ?

Probably, or just give it max HP instead of rolling.
The other part is tactics and traps. If they are in its lair, it should have traps, glyphs of warding, and other things to protect its hoard when it's away.

 

[dave2008]

I get what you're saying, but I would advise against a solo combat the ends a campaign that does not feature legendary resistance. A single failed save ending the climax is ... boring.

Sure, but in theory legendary resistance is figured in the CR (30 "effective" HP per use). So simply by adding legendary resistance you increase a creatures CR. Also, a legendary creature has legendary actions, but not necessarily legendary actions. In fact the very fist Legendary Monster in the MM, the aboleth, is a legendary monster without legendary resistance.