I think that U.A. monsters are really good
(especially blockbacker and slugger)

one problem is that i think they dont have enough support
and dont have enough lvl4 monsters (only midfielder)

in order to get rid of this problem I used:
some lvl3 monsters to summon up M-X invoker
which should summon midfielder (starts off plays)

Could there be any more ways to improve this deck?
i really see huge potential in this archetype

I had the same problem I got over that hurdle by using a lite burning abyss engine, only need 1 tourguide (after april 1, 2 if its ocg) 2 scarn (to search) and 2-4 different burning abyss cards (just 1-2 each), they all can be special summoned from the hand to either to be used for rank 3 or to be advanced summoned. by doing that can steam roll almost every deck 4 of 5 times (my version anyway with said method).

even with that method HAVE to go for the otk method there is a similar post about UA look into it

I would suggest using RotA to search Midfielder, and a summoner monk or two. It seems excessive, but the deck is so dependent on pulling them out that it's worth it