K12

Your board is a K 12 Pro with white backlight, hot swappable, mechanical red ... link. It is not compatible with optical "switches". This is a good thing, optical "switches" are obsolete and a dead end technology. Why do you think you want optical? They are no quieter than mechanical switches, since

The Keychron K12 is not a QMK board and has no configuration options. If you need that functionality... can you return it and get a Keychron K12 Pro or V4 or one of the other QMK based 60% boards currently on the market, like the Skyloong GK61 (Pro) QMK/VIA or Redragon K717 Alcor?

keychron k12 with their silent switches might work well in your budget, i've been using one for work meetings and it's quite good for the price. just make sure whatever you get has good wireless since android can be bit picky with bluetooth keyboards sometimes

Folio case K3/K3v3, K3 Max, K3 Protravel-pouch, fits80% - Keychron K1, K1 SE, K1 Pro, K13 Pro75% - K3, K3 Pro, K12, S1100% - K5, K5 SE, K5 Pro65% - K7, K7 ProK9 Pro or carrying-case, fits a lot weight, some are bit more, some bit less than 600, think you've a decent choice at keychron

Is it different in wired mode? Make sure the keyboard isn't in (full) NKRO mode (it happens inadvertently much more frequently than one would think). It completely busted my K10 Pro keyboard; in fact, the symptoms were pretty close to the symptoms you described. A simple test for (full) NKRO (do it

What keyboards? They aren't created equal (Q9 is an extreme example and K12 is somewhat less extreme).