I would agree with you. I actually really like Colton Haynes and Keiynan Lonsdale as people outside of their portrayals of those character. I definitely think they have been held back by the writing. Wally's portrayal is a bummer in particular. He isn't living up to his potential at all. The portrayal needs more kinetic energy like what we've seen in the comics and in animated portrayals. He feels far too similar to Barry.
I was honestly not that disappointed in Harper's portrayal on Arrow now that I think about it. It felt pretty accurate
Only issue with incorporating these two characters into Titans is the whole different universe thing. Titans is set in a universe where Batman is well established, and so far Batman has only been referenced sparingly in the Arrowverse. I would love to see Haynes' and Lonsdale's Harper and West tbh. I started thinking about it and neither of them have done a bad job. They'd be welcome additions to this show, but if they are recast, I won't be upset at all.