You should spare yourself some of the guesswork and run 6-8 peek effects - Duress, Erasure, Sorcerous Spyglass. Sometimes it can be difficult to distinguish between the more exotic shit people play
(I once named Teferi vs. a deck which looked like a mana screwed esper, turned out to be a non-blue Outlaws Merriment).
Is mono blue counterspells even a thing? If so, I wouldn't go for the counterspells, but its win conditions and card advantage engines.
For UG Flash, you want to remove Nightpack Ambusher because it's the main win condition. Give it a quick browse as you neuter the wolf,
sometimes the secondary wincon is Cuttroat, other times it is Wildborn Preserver. Can be both. Also, some faggots started to run Oko.
Once you take out the main and secondary wincon, the rest is pretty benign.
Lucky Clover gets a lot of mileage out of Edgevall Inkeeper, though not sure if its feasible to Ego it, because it's most valuable when dropped early and you should have removal for it.
For Fires of Invention, removing the titular card might force a concession, depending if the pilot is comfortable running the deck without it. As you browse the list, make note of the win conditions.
It's a bit trickier if the Fires have already landed, Fae isn't a terrible idea cause the wincons are usually distributed between Kenriths, Sarkans and Chandras, some might be tucked into the sideboard, see what you can punish best as you browse the deck.
There's the Cavalier variant as well, in this case you still start with Fires, then I believe Cavalier of Flame is the more dangerous one.
When it comes to aggro in general, I think Ego is weak here. You lose tempo and the shit that's killing you is already on the board.
The priority play is to wipe the board reliably on turn 4-5, so if you want to Ego something, I think I'd start with shit they didn't play yet that can still finish you off afterwards.
E.g. a topdecked Torbran gives you an entire turn to react, and Cavalcade does nothing without a creature. On the othe hand, Slaying Fire immediately hits you for four which can be more threatening.
Yeah, Questing Beast is probably the most annoying thing in green-based aggro.
