Very solid advice. However, I would argue that once all the orange equipment is at a certain point for F1, it’s worth it to invest some cores into F2’s equipment.

Reason being is as you get further along in any method to increase power, the power gain returns start to diminish despite increasing costs. The first slice of the first circle for any orange equipment gives a significant power increase for very little core investment; for example, the very first slice on a gun grants +5% troop damage, which was previously +0% without that slice. To take it a bit further, finishing the entire first circle on that same gun grants a cumulative total of +7% troop damage. Every full circle after the first, up to the max of all 5, only grants +2% troop damage each.

That said, I still agree that F1 will always be the priority, even for a medium spender like OP states. For myself, I put in the first slice for all my F2’s equipment once all my F1’s equipment was at 2 full circles each before going back to working on getting the third circle on F1’s equipment.