Hey Monardo welcome to the forum
The belay loop is the only safe point of attachment on most rock harnesses
It could get a bit cluttered on the belay loop, you would have to be extra careful disconnecting biners, say have a completely different Biner for your tether
On rock a sling girth hitched to the belay loop with a Biner on the other end works well as a tether, I'm sure you could adapt the concept to trees
Let us know how you get on
That could help as it adds attachment points in a clean way :
This one cost around 27$. I guess you can find sales on it or other less expensive products but it is allready a low 2 numbers price.
I'm not a big fan of webbing lanyards as using rope is easy to build an adjustable system. To me webbing lanyards are the way to go when you don't have any other solution.
Adjustable lanyards are more than confortable, it really helps to perfectly adapt your attachment to the tree. Plus you can add a strangling system, so even if there are no limbs to retain your lanyard it wont glide along the trunk. Things you can't do with a webbing.
Two figure eight knots, a friction hitch and there you go, you got an adjustable lanyard. Add a cheap micro pulley to have more ease of use if you wish, but that's not mandatory.
Using a lanyard is great. It brings safety and mobility. But it becomes a bit technical when you start to do your own rope system. Taking a tree climbing course would help you a lot to know for sure how to do it perfectly, and many other necessary knowledges.