on the one hand i agree with what you're saying, but when you think about it almost none of the actors were sci-fi or technically savvy with the exception of GW, TR, or RP. when you look at someone like RD who came in with a dim view of sci-fi, she handled the technobabble like a champ. almost everyone did. i think it came to the underlying attitude of how you perceived both your job and the genre itself which RB has said he loathed then and now.That's a different issue all together actually.I think they scaled back Chakotay and Kim for behind the scenes reasons. Beltran couldn't read his lines and was holding up filming on reshoots & Wang wasn't taking his job seriously. So the production team wasn't going to waste time writing for actors that were costing them money.
I think Tuvok & Neelix got knocked back due to popularity.
Neelix has a love/hate following in Trek fandom, so why put too much focus on a character some compare to Jar Jar.(However, I think it'sa crime how fans overlook Ethan Philips' performance in "Once Upon a Time".) Tuvok I think just drew the short straw. IMO people seem to overlook him due to the surrounding characters being more "colorful" in their personalities. The writers knew this and scaled him back too.
Tuvok's dry wit is greatly underrated as is his beautiful and comforting words to Ensign Wildman about Niomi's future in "Once Upon..." as well.
Beltran knew all the lines for Hamlet (from Tim Russ’ anecdote of Beltran and Ray Walston), so it wasn’t like he couldn’t learn his lines. It’s been said that the scripts were constantly being changed up to the last minute. Whether this was a production decision, writer decision, or to compensate for an actor’s disinterest is unclear. But I would agree with what Beltran (supposedly) said: Why learn lines that were going to be changed? Learn the final ones that mattered.
The cast and Beltran himself admits that he would stumble over his lines the most because he had trouble saying the technobabble. This problem goes back as far as the first season of production. This is even before rewrites in the script became he next issue. If you watch his performance in the show, you can even hear Beltran hesitate in saying certain words or phrases. Due to it, many retakes had to be done when filming scenes that gave Beltran a speaking part. Star Trek sci-fi was just the wrong genre for an actor like Beltran.