Hiya
April and May and the best months for finding the bigger fish with their guard down.
You'll get them all year but it is easier early on.
Public transport will limit your access to the river unless you stay somewhere like inverurie and fish the council beats, kintore and alford are on regular bus runs from the town and you can walk to river easily in each town. Some great fishing on all those beats as well and lots of accommodation.
The upper river has hotels with beats as you say. Lots of great water to explore at kildrummy and monymusk. Monymusk is easy by bus, kildrummy less so but the hotel might have a solution to that if you ask. Both beats are big enough to keep you entertained for a few weeks.
Cheers
Sandy