[perko90]

nullve, on 2021-May-18, 06:22, said:

With 18-19, 5D(332) I'm going to declare NT many, many times for each time it will be useful to show my fifth diamond in competition, so I'd rather open 1♣ and risk playing in a non-fit, just to avoid the information leakage and be able to stop in 1N.

I've used that treatment before when I was playing a simpler 1♦ structure and needed the 2NT rebid for another purpose. It worked ok.
But as long as I can fit it in, I like opening 1♦ when I have 5 of 'em.
It's not just for competitive situations. I'll also be much happier if the auction goes 1♦- swish. And there's also a possibility of finding a nice D slam that others may miss.
Also, while you often get to rebid 1NT with the big NT hand by starting with a short club and getting a T-Walsh response. There are sometimes where you have to rebid 2NT, too. (ex. 1♣-1♠*). Here, too, it's nice to rule out the possibility of a 5-card D suit.

[nullve]

perko90, on 2021-May-21, 22:34, said:

Also, while you often get to rebid 1NT with the big NT hand by starting with a short club and getting a T-Walsh response. There are sometimes where you have to rebid 2NT, too. (ex. 1♣-1♠*).

Not if you use the trick mentioned by helene_t many years ago:

1♣-1♠: does not contain a BAL invite opposite the split range 12-14 OR 18-19
1♣-1N: invite opposite the split range 12-14 OR 18-19 (Opener rejects/accepts with 12-14/18-19, respectively)

Then Opener can comfortably rebid 1N over 1♣-1♠ not only with 12-14 BAL but also with 18-19 BAL.

I hereby declare this to be part of Standard T-Walsh.