This is a beautiful country. Avoid July and August and last week of June if you can - school holidays here are six weeks from more or less start of July. Masses of French and German tourists among others come via plane/hired camper or car or via Newcastle. Passing place etiquette on single track roads is not always as it should be! Don't let the midges get to you!! My mantra to self is about the lack of actual harm they do. You may look like you have measles and may itch a bit, but really it's a lot better than ticks, or cleggs (horseflies etc) come to that.... I'd go for first/second week in June for plants (we are normally a bit behind) and weather?
The nature reserves of NE are a good shout from Derek - eg Muir of Dinnett SSSI (H16, H16a), Parkin's Moss (M18, M19, M4) are both within Cairngorms NP, on the Aberdeen/Banchory/Braemar side. Ditto the wonderful Corrie Fee NNR. NE is definitely drier. Accommodation -avoid high prices and pressure of Spey valley? But places like Kincraig and Insh are great as they're off the main road, nice rentals though booking pressure is definitely there (and def need car).
Mike is of course correct, as is Derek - tourist numbers are on the up in all sorts of places. I work part time for the other NP (not Cairngorm) so I am out and about most of the summer. Yet there are places even in the Loch Lomond and Trossachs area where you can find some peace and hopefully some plants, but you do need your own transport and to walk in a bit. An example: Glen Finglas, part off the Great Trossachs Forest NNR. Plantlife Scotland have fact sheets on IPAs https://www.plantlife.org.uk/uk/nature-reserves-important-plant-areas/important-plant-areas and http://www.plantlifeipa.org/site/factsheet/1106 but you'll know that. They don't list the nail bars.... The west has fabulous oak rainforests eg Glasdrum Wood, dripping with lichens but then the weather explains that vegetation....
Scottish protected sites, why they were designated and most recent condition monitoring etc etc can be seen via SNH's site or on Scotland's Environment Web eg https://map.environment.gov.scot/sewebmap/?layers=eunisLandCoverScotland&extent=-798759,60340,1333758,1722563 where land cover and SSSIs were both selected (don't know if link will work!).
Anyway, good luck with the plans or thoughts and keep the questions coming! Mags