Trying to boil down your first initial requirements from what you have said so far ... forests, high trees, surveillance, perhaps not so much filming or photo shooting.
Coming from my own experience I know that a drone with enhanced wifi (like Mavic mini, Mavic Air, Spark & Parrot Anafi) are prone to connection difficulties when flying out of LOS (unobstructed
line of sight, & I'm not meaning visual line of sight).
So ... assume a clearing in the forrest with 100 feet radius, 160 feet tall pine trees all around & the drone on the maximum allowed altitude of 400 feet. Then you will already loose the LOS approx. 240 feet horizontal away from were you are standing & connection cutouts can start to occur. So your surveillance area will be rather small.
If my assumptions are somewhat correct I would go for a drone with Occusync connection (
Mavic 2 Pro/Zoom or Mavic Pro) just to get the perhaps more reliable of the two..
Camera wise you could generalize a bit and say that It's kind of 3 levels spaced rather far from each other ...
Mavic Mini + Spark:
Beginner level with several missing things/functions or a quality level (of pic or video) that will make you wish that you bought something better if you have a camera interest and want to grow.
Mavic Air + Mavic Pro + think Parrot Anafi is comparable here also:
Medium level with good enough camera result level with some room to grow, nearly full chart of functionality like 4k video, RAW stills & a lot more. All this spiced with tons of autonomous flight modes assisting you to get that cinematic & smooth look.
Mavic 2 Pro/Zoom:
Pro level with a even more refined camera quality, lower noice levels & wider dynamic range.
Battery or flight time capability ...
Well more is always better but what's enough? In practice (generalizing a bit again...) you can stay airborn for approx.10 minutes with the Spark, 15 with the Mavic Air & Anafi, 20 with the Mavic Pro & Mavic
2Pro/Zoom and then 25 with the Mavic Mini.
For instance so have I 4 batteries for my Air which is more then sufficient ... of course it depends on if a return for battery swop is an obstacle or not.
Sensors:
First of all ... you can never fully trust them, they aren't fail safe. Small thin branches can be missed & you crash anyway.
Both Mavic Air &
Mavic 2 Pro/Zoom are
full of them ... but you should always go the extra mile to ensure that the flight path is free anyway.
Mavic Pro & Spark
have them to a lesser degree, and the Mavic Mini
are free of them (don't know how the Parrot are equipped).
Portability:
Pretty easy ... the smaller the better. But it has some drawbacks like how well they take high winds. Mainly up to you how of a compact package you like to carry.
So in the end ... no one are the best of everything, you have to pick that which tick the most of your "must" needs" & a few of your good to have's.