Yes, I think that Avant is clear. It will probably remain competitive and you won't need to switch again.
You could switch a few times to max the cash back, but for a simple mortgage, go with Avant.
Don't worry about it being a fixed rate. The way break fees are calculated, it's very likely that there will be no break fee for breaking early. Or if there is one, it will be quite low.
If competition hots up and some other lender offers 1.4% fixed, then you could switch.
Brendan