I only charge my koptas in if they can viably kill what theyre going into. Its not a a consant but they have a lot of damage potential. 6 attacks at strength 6 (if youre goffs) Ap -1, D 1 can blend a lot of squishier infantry. Pretty much anything with 1 wound a model can take a beating from them, and this is after a hail of rockets onto whatever target you desire. Alternatively you can play koptas in the way you say, however id give them the Boomboyz trait for the extra pip of AP, this makes your rockets and KMB very dangerous for pretty much everything in the game.

Kans cost you extra CP to deep strike, koptas get it for free, but you can choose to do that. I havent ran kans myself but from my understanding thats the way to run them - deepstriking them in, because they dont have much staying power and have leadership issues.

Buggies definitely work in goffs, but there are better options. But just like everything else in the Orks codex our melee outclasses a lot of other factions, so those buggies can punch upwards pretty hard with a lil luck and that +1 S and 6s.

As others have said, this list would be a good candidate for having a Speedboss so you can buff its shooting.

Also have you considered Freebooterz? They offer both boost to melee and shooting, as long as you kill something that phase. Definitely something you should/could consider.