Welcome to the forum,
MAVIC3 is an awesome drone, congrats...
Definitely a yes to spare batteries and a car charger, and a better case from me - My
Air 3 and
MINI3 Pro both have rucksack typs cases so I can carry them comfortably for some distance, our
AVATA has a more hard case design...
Whereabouts in the Highlands are you? We are based down in Manchester, but come up to Scotland 3 or 4 times a year, last trip was Skye and Lewis and Harris, next is possibly Tongue in Feb and then Blairgowrie in April and Skye again in June...
RAF low flying area covers a large amount of the Highlands, but is not always in use, check the RAF schedule and if in doubt then ring the RAF Low Fly team, and they will let you know if you are safe to fly..
Military low flying: RAF operational low flying training timetable
As to editing, I am a huge fan of DaVinci Resolve, free and very well featured, however a steepish learning curve...
Good luck, looking forwards to seeing your captures...