I can see that everyone has given you lots of advice on here already!
If you have managed to take all that in – then re Stronghold – I dosed my dog with it last year and ALL his guard hairs on his coat fell out! (Poor thing had no waterproof coat all winter) In the side effects it notes hair loss in cats, so it’s not implausible to think it might cause it with sensitive dogs.
Have also recently had to take my dog off Royal Canin as he seems to have developed food allergies. It isn’t a great food, and since he has been off it his skin is slooooowly improving.
Any skin stuff is hard to deal with, but the advice you’ll get on here is second to none.
P.s. can take up to 2 months for hair to grow back, so whatever you do be prepared for a bit of a wait!