This may actually be a great application for a film retrofit. I’ve ordered a few samples from aliexpress, and the two issues I had were: 1) optical clarity is good not great, and 2) not dark enough for my needs at maximum opacity. for an indirect window like this, that may not need to be very opaque.

Electro dimming (electrochromatics) is not rocket science, but it’s not something you can buy off the shelf either. Even if it was available , it would be a total window replacement.

For triangular and oddly shaped windows, OP should look at Venetian blinds because they can be cut to any shape without affecting their operation. there are also prebuilt home automation products that can turn the operating screw of Venetian blinds.