Realistically, will it be safe to fly to and travel in the Benelux countries in March/April 2021.
You have to track developments in those countries over time and see what happens - then make your own decision based on your health status and financial and psychological risk profile. No one here can answer your question with any degree of confidence this far in advance. Airlines can't even plan their schedules this year, so that's a good indicator that things are very much in flux.
The long and short of it I’m afraid is that nobody knows. Nobody here also knows your tolerance of risk or how vulnerable your personal situation is.
Everyone will have a better answer is another six months. Till then who knows. Very dependent on finding an effective treatment plus a viable vaccine. I would plan it but make no major financial commitments.
I don't think your health will be the main issue, but whether you can do what you want to do. There might still - or again - be strong restrictions on restaurants, cafés, museums and whatever, or not. Nobody can tell. I would indeed wait and see.
Planning is half the fun, so why not go ahead and plan it but without any significant financial commitments. Then, if the answer is no, you can cancel it and reset the dates for 2022. I'm about to pull the plug on our plans for this Sept. and will reschedule for Sept. 2021 with my fingers crossed that 2021 will work out. Accommodations and the lease car are easy enough to do on a refundable or minimal basis, it's the flights that get hairy. The big problem with the Covid situation is that the more they learn the more they don't know. Good luck to all of us.
Good ahead and plan, but personally I wouldn’t make any reservations. With the current airlines’ schedules changing all the time, and with all the cancelled routes, the only thing certain is change. My flight to Spain later this year (if Spain is even open by then) has changed three times in the last month.
If by "planning" you mean reservations, I would do only those that are refundable/no prepay.
I hope travel restrictions are lifted by then but realistically it’s impossible to know at this time. I hope it won’t be necessary to isolate for two weeks on arrival - building that into a trip plan seems a bit crazy - like bring a good book :-} or two.
I’m hoping our lost 2020 trip can just be slipped forward one year and we will ski in the Berner Oberland and under the Matterhorn in March 2021. Then go to Keukenhof instead of just watching videos!
Currently, I feel 2022 is a lifetime away and it’s just too depressing to think we’d have to wait that long.