F-35 isn't really on offer to us. Trump's words mean nothing. We will never buy it because we are not seriously considered as a customer for the F-35. A formal offer has never been made and likely won't be until the end of the decade when they start rolling out their own sixth generation fighters and many more fifth generation fighters would either be in the market or will be shortly.
As for the Su-57, the infamously import loving IAF itself openly criticized the project a decade ago. The Russians themselves operate approx 30-40 Su-57s with a third of them being established prototypes. They apparently ordered 78 of them but deliveries have been taking forever for the main customer itself, when do you think we'll get our deliveries if we order 3-4 squadrons tomorrow itself? The Su-57 definitely won't be a stop gap, it'll keep getting delivered right as AMCA is finishing the flight certifications in mid 2030s at the pace they are working on. Mind you, we will likely be immediately sanctioned and there's a possibility of French sales being heavily curtailed in the time it would take for the Su-57 to be delivered. Is the bird really worth all that strategic trouble?
If Pakistan acquires J-35 it would be a blow to us but that shouldn't make us act all hysterical and irrational. The MoD, DRDO, HAL and IAF should do their best to stick to the timeline and be willing to handhold projects into maturation instead of changing requirements mid-way through development and cause further delays.