I'm already playing at the same level as I did with a mouseThis is completely normal.
The peripherals don't matter as much. Just think please.
A lot of your skill lies in your tapping hand and in your eyes and in your brain.
When playing on an easy level of gameplay(and you certainly do) there isn't real muscle memory you can build up in terms of aim cause every map is just distance-snapped 1/1, 1/2 slider-circle spam.
So effectively 95% of your ability transfers over.
Also aiming - which is a big hurdle for mouse-users at the beginning - is much easier with tablet but you will eventually run into trouble with your cursor-stability which is a problem mouseplayers simply dont get.
Personally I would suggest you to download some beatmap packs as you seem to play only very few maps again and again which generally hinders improvement as you start to memorize the maps unconsciously.