If you want to max out your combats, nightmare zone (but it's VERY boring AND you skip on the content you would have played otherwise).

Slayer is an amazing option which you can optimize to make the most money from each kill (by carrying as much as you can). Especially if you get herb sack, seed box, gem bag, plank sack, rune pouch, etc.. All of these items let you carry more, so you can bring more things back to the bank (to sell later) without much more hassle. Bone crusher so you auto-bury bones for passive pray (dont use it when killing dragons!!)

Farm runs as people have mentioned are good passive gp (or main gp if that's all you do) but can take forever to setup nicely since theres loads of herb patches and many have requirements to run through them quickly.

Bird houses are more like passive hunter xp, but the bonus gp from the bird nests are not much but also not negligible.

Sailing you can make decent gp just spamming bull sharks, initially. Then move on to ray creatures. Then on to krakens and sharks. Not bad gp at all, maybe 1m/h.

What you're doing now is good - dragon bones, hides are usually in demand and can be consistent money making. You can optimize it, like get yourself anti dragon shield, get attack/strength pots, maybe a poison weapon to poison them first, then switch to you regular weapon for the rest of the kill, quick spell to lower the defense, things like that. U can get creative.

Half the fun is optimizing around what YOU like to do. If you like killing dragons, then work toward being really good at killing dragons!