@txav8r , sounds like you are ready to go all in on this as i usually do with new hobbies. My suggestion is that you wait to buy more accessories. Most of the accessories i bought get no use.
I have an nd 8 filter but dont like it because it snaps instead of screws on, ive also never really needed it but some may disagree when they see my footage. I run the stock settings by the way.
Float balls work but slow the heli down a lot and probably hurt flight times as well.
Extended gear is cool but like other add ons such as the prop gaurds they dont allow the mavic to be folded up for storage or transport.
I have a bunch of sd cards of varying capacities but found that the 16gb that comes with it is fine, i dont record in 4k so the 16gb card will hold at least an hour of footage, maybe more. If you run 3 batteries consequetivley and are flyjng not just hovering you wont be in the air for more than an hour and still have 20% of the charge remaing per battery when you land. An hour is still a long time for me and ive accepted that i risk losing or damaging the $1k heli everytime i put it in the air.
You can set the app and/or phone so that it doesnt record the footage to the iphone. That is how mine came set up out of the box but when i use an android tablet with removable memory it records the footage i take, i think in a lower resolution than the camera takes it in.
As for must haves, an after market gimbal protector is nice because it locks the camera into positon unlike the stock unit which just covers the camera and gimbal. I tried to use one that had a cut out in it so that the camera and gimbal are protected but can still be used in flight, its edges kept getting in my footage so i stopped using it. Another good updgrade is the rc control stick/display protector. Lastly is the getter back ballon float because i fly over water a lot. All of the these can be found on line for under $15 each.
I have not bought the new blades yet and probably wont. Not because i dont want them but because i dont need them as i have a full set of spares and have yet to replace the original blades. Plus i usually dont fly where noise would be an issue.
Big differnce betwen a no fly and restricted zone, no fly will keep the bird from spinning up or will prevent a bird in flight from entering the zone. Restricted zones result in an on screen warning and a request that you confirm and accept the risk. These have been coming up more often for me lately. THey are a hasle because the bird will stop and hover at the zone border until the user confirms and accepts the risk but they are managable.
You dont have to recaibrate the compass every time you fly. I only do it when the app tells me to. I have had to calibrate the imu once to resolove an out of focus issue but that was about it and firm ware upgrades have pretty much taken care of the problem long term. I still set one of the buttons on the back of the rc to focus and hit it whenever i notice the view is blurry (auto focus is good but not 100% effective).
If you are visiting state or federal parks check to see if they have an anti drone policy. National parks dont allow them (actually any photography) and a foreign user found out the hard way a few years ago when he crashed his drone in the water at a national park. The park service gave it back to him after he paid a $2500 fine for taking photgrpahs /video in the park. Seemed like selective enforcemnt to me but i would not chance flying in a national park.
Ive seen a lot more no drone signs over the years so i dont fly at those locations, near airports, hospitals or anywhere that i feel i have to hide what im doing. If a spectator feels uncomfortable i will usually stop until they are gone or will fly somewhere else they are not. Most are just curious and want to know more. The exceptions being those few non leos that feel they can make or order me to stop. I just keep flying and either ignore them or have a discussion about legalities with them. In the past with the conventional pod and boom helis ive put the heli in a hover between me and the agressor when necessary, that always shut them up amd made them back away, justifiably so, with blades 550 to 700 mm each spinning at 1,500 rpm. Please still goes a long way with me.
Anyway, sorry about the long post, hope it helps, as an aviator i dont need to tell you not to be one of "those guys" that give the hobby a bad reputation but be aware that there are a lot of those guys out there, especially now that the cost of a complete system is under $1k, they run on electricity not gas or nitro methane and they are easy to fly/hands free hover with four to six counter rotating and small blades.