The average trainer will do a lot more than most people think. It can complete all maneuvers in the AMA "Sportsman" Pattern competition aerobatics schedule well enough to compete with it. (and I've seen trainers win...)

If you can't do a roll withthe trainer yet... you probably are not really ready for the next airplane. Rolls are not hard. (just scary till you understand them) The trainer, as it goes inverted wants to drop the nose toward the ground FAST. If Take it high and practce doing 1/2 roll and giving full down (or a little less with some trainers) to prevent the nose dropping. Then do the other 1/2 roll. Do the 2 halves closer to each other each attempt... and you will soon find the roll rate where you can make it look like its a continuous roll. (then it won't be much longer till you actually have it continuous... the plane often tends to "snap" through the small portion of the roll where you are applying down elevator.)

Trainers will ALWAYS need significant down elevator to fly inverted. You can;t get the CG back enough to fix it. The reason is "Decalage" (the angle betweent he main wing incidence and the tailplane incidence.) With an aerobatic design, decalage will be 0 or very close. With a trainer... it can be 5 to 10 degrees. That is why trainers ALWAYs pull up out of dives on thier own (if there is enough room)

Moving the CG back can help with aerobatics... but don't overdo it. You get it too tailheavy and it becomes virtually uncontrollable.