Central banks do not issue bonds. What they've been doing is buying bonds issued by national governments. Usually on the public markets, not directly. Where do they get the money to buy the bonds? Why they just magically create it. That's their job.

Central banks do not care, per se, if they have to "pay incredible amounts of interest to the banks" since they literally create (and destroy) money. Central banks do not need to buy or sell gold to create money since money is no longer gold backed. Operationally, central banks can create any amount of money they want anytime they want. Obviously, there are political considerations though.

"More likely is they allow fiat to devalue", yes. That is exactly what they will do if inflation increases substantially. Though not because it will make their loans "loans easier to pay off" since central banks have no loans to pay off; instead they get paid coupons and principal on they bonds.