Hi
Can only speak for Jet2 and TUI. They have been releasing the dates early, I assume, so as to get some cash flow in these difficult times.
We have been booking Jet2, flights only, to Corfu the past few years very early, as soon as the flights are released. For example for Sept 2023 we booked Oct 2021.
Never had any problem to date. Get, I think good prices.
We book package holidays with TUI to Skiathos as equally early. We have been stopping with the same family for 27years. I ring them up to confirm they will still be with TUI prior to making the booking. Never had a problem. So far.
Geoff