From the 26th April it's a free for all, especially at the weekends.
Totally unscientific study but just going from last year when lockdown eased and at weekends. I arrived at the North face car park at 7am on a Sunday and had to park
sensibly just outside the car park.
Glen Shiel was also close to full at 7am on a saturday.
Some spaces unfortunately are taken overnight by campervans and at the NF some inconsiderate campervan parking also.
Glen Muick car park is very popular also but a fair size so hard to tell really, but I would try and be there at 7am at the latest or go somewhere less popular or at least have a plan B. Another problem at Glen Muick could be getting out the single track road at the end of the day if the volume of cars is not monitored.
Of course the poorer the weather the more available parking.
All the best