All those controls, rules, policies and requirements keep changing -- as they must because of the biological ability of COV to keep shape-shifting.
Try checking 6 weeks/4 weeks/2 weeks/ 72 hours out from when you aim to go. The information will be more valuable. Information now is rather irrelevant to that far distant future.
I am hoping I can acquire whatever MRNA booster will be on offer come early winter, and then fly to Spain for Jan 1, 2022. Will it be possible? No idea. I don't need a VISA for entry so I was able to buy a one-way, very cheap ticket that I can afford to lose.
I would tell any friend what the loan lottery corporation tells us: know your limits, play within it.
If you can't insure your ticket, or can't afford the loss, then I'd avoid booking now.
Keep your eyes open for the ticket sales, and for the virus trends in Spain and around the globe. Watch the hot-spots where you will have to make lay-overs (Dubai? Bangkok?), and makesurea you get health insurance to get you home if you need it.
And if uncertainty drives you to distraction, or having to meet a bunch of deadlines with moving goalposts will ruin your trip, then I'd say: try holding off until a time when it won't be spoiled for you.
Other than that: Buen Camino.