Dura-Ace 9000 DI2 rear derailleur max cog size

Shimano says that 28T sprocket is the max I can go on my DA 9000 DI2 rear derailleur. I understand that it’s “not recommended” but I like to climb and am getting older. Will this derailleur work effectively with a 30 or 32 cog sprocket? If not, are my only options a different DI2 derailleur?

30T should work just fine… If you’re feeling brave 32T will probably also work, but don’t blame Shimano if your derailleur explodes and kills a kitten.
Also, shifting won’t be as smooth if you go all the way to 32T…

In terms of compatibility… you can use any of these (assuming 2x):

RD-6870-GS, RD-R8050-SS (30T max, can probably do 32T fine), RD-R8050-GS (34T max), RD-R9150 (30T max, so not ideal), and the GRX: RD-RX815 (34T max, and clutch), RD-RX817 (42T max, “1x only”. Will work with a front derailleur installed, but won’t do synchro shift).

RD-R8150, RD-R9250, RD-M8050, RD-M9050, won’t work (they’re either 12 speed or mtb)

32T works fine on my bike. Replace the B-Tension bolt with an M4 bolt with an Allen head and turn it in the other way round so that its head safely rests against the “hook” of the derailleur hanger.
I then use a nut to secure the bolt against the derailleur to keep it from turning over time.

If you want to go even (much) bigger I’d recommend using the GRX RD-RX815. It easily manages to shift the 11-40T 11speed cassette and is fully compatible with the rest of your bike’s drive train.