"Something strange about these graduations is that some girls (Aya, Moe) get a proper send-off on screen while others (Anri, Shunka, Nami, Ai, the Usui sisters, Nana and maybe Rirei too) just... dissapear without any mention about them

"
Yeah, PhantomJin, I agree...it is strange. I think it was the same with the OG Muscats. Some girls got the full send-off, some just quietly disappeared.
Perhaps, it's a scheduling thing.
It seems likely that the program is recorded once a month.
It could be that the production staff need a couple of months notice to organise "a proper send-off on screen".
Shunka, for example, might have quit the program between one recording day and another.
I can only think that some of these girls wanted to slip quietly out of the program.
That they were offered a on-screen send-off, but turned it down. That would explain why their leaving is never mentioned.
Both Rio and Hatsune Minori quietly slipped out of their AV careers without a big send-off.
In contrast, Kasumi Kaho made
100% sure everybody knew she was retiring from AV.

Of course, there is so much 'stuff' that happens (both with the lives/careers of individual Muscats and the production of the program) that we do not know.
It makes it difficult to ever be certain about why this, or that, thing happened.