At present, W10M isn't, currently, available for the lovely 720. This, if I recall correctly, is down to the 512mb memory on board. If you have tried the insider builds, as you state, then you will probably be aware of the issues. Glitchy, slow, struggling and crashing.
It's a sad fact, but MSFT have put a cut off in place for older phones or those with 512mb. Don't give up on W10M just yet. I'm not sure where you are in the world, but in the USA, there are some good deals on the cracking 640, which runs W10M fine. They have been available for less than $40. Another 'budget' phone is the 550. This one comes with W10M straight out of the box. If you are looking at the HTC 8*, then you will be disappointed again, as on the 29th April 2016, HTC said that there will not be an official W10M from them, so you are looking at insider again.
Sorry to be the bearer of bad news,
If you need more information, research the Microsoft 640, or any other devices that can run W10M, why not join the community forums, and pick the brains of the members?