Fantastic Sydda :) His herdboundness is something tough for a trainer to fix as I'm sure the minute he's back with you he'll go back to his old behaviors. He's acting rude with you alone, wanting to be back with his friends. Horses do this because other equines= safety, he doesn't yet see you as something to provide him safety.
A few ways to earn this is respect building exercises, particularly joining up. you don't need a round pen for this!
It helps, but if you have a small pen, large enough for them to run around in but small enough for you to keep up, you can do it.
I think having a trainer would be great, but you might be better spending your money to have the trainer come and teach you how to work with your horse. If the trainer came in and worked with him once or twice at your property I'm sure he'd be fine for them, you
need to learn how to handle the horse, the trainer already knows and the horse already knows, he's just taking advantage.
Ask your parents if you can have a trainer come help you instead. This way you and your horse both learn. It may also be cheaper.
This new location sounds perfect for you and your horse. I'd look into that.
As for CT, don't be ashamed to say you use treats, it's not bad so long as your ONLY reinforcing behavior you want to see more of. Him walking politely on lead, reinforce, him trotting when you ask, perfect time! Him invading your space or being rude = Big No No! Once he knows that click=treat (or whatever noise you use) you can transition it into riding.
Work on each skill on the ground, giving to leg pressure (on the ground using your hand where you leg would be) and giving to the bridle (on the ground use the reins like you would). Then transition it to mounted you use your click to mark the skill you want, stop him and treat him (by leaning forward and letting him get it out of your hand).
Here's a video of someone clicker training while mounted so you can kinda see how it works:
Again, please don't feel like you need to use it, you can train your horse however you like. But he sounds to me like a sour horse who needs some respect and something to make him want
to be with you and want
to work for you. He's not like many horses who are just 'happy to please' for a relief of pressure, he seems to need a little more 'motivation' than that.