Power up dragonite?

Yes, and yes 👍

Yes!! You won't notice much of a difference between that Dragonite and a 100%.

Yes, you should be using it against giratina and will be very useful against other future dragon type raid bosses.

I have a much worse one powered up back from the days when TM's didn't exist and it was my only Dnite (wild catch). It's a very, very safe investment.

According to the CP calculator, your Dragonite is currently level 26. Powering it up to level 30 will require 32 candy. So go ahead!

I would wait for the next community day announcements. Since bagon might be next

Yes. Power him up at least to level 30.

It's good enough to power it up to lvl 30, it's very useful, and it won't cost you a lot of stardust/candys.

It's good enough to power up to level 40, but then you need more candy ;)

15 atk and over 80% Ivs is always worth powering up at least for meta relevant pokemon. And specially since it's your only one. Getting a high IV Dratini from research and then evolving and powering it up may cost you over 200 candies!!.
Yours is perfectly good and is already evolved, power it up to at least lvl 30!

of course! Just keep in mind that there is rayquaza's return (highest neutral DPS pokemon) and also powerful dragon types like dialga( a dragon not weak to dragon), giratina origin form(a big bulky dragon), garchomp (outclassing dnite as the best not legendary dragon ) and last but not least, palkia with that glorious dragon and water type.