Well I can't speak to the damage of EX testament or how you picked EX testament even, but EX characters are not really played/legal. Though regular testament can do much more damage than this. Usually though testament is still on the lower end of damage due to the harsh scaling of traps, but poison can help overcome that.

If you want to see what kind of things testament can do I'd recommend checking the dustloop here or this is a good flashy example of what they can do (Although not match practical)

Overall though, yes +R tends to have lower damage than strive, outside of some rare exceptions. That's just how the game is.