Where are you staying, that would go far in deciding on what park to do that first half day. I think for the shorter trips park hoppers are your friend. The level of planning really depends on your goals. Do you want to ride as many rides as you can or do you want to just ride a few select ones but soak in the atmosphere in certain parks? You say you want to see Pandora in AK and the new stuff in HS. You might need to be more specific about the HS part, almost everything in that park is new as far as rides and lands from the last time you were there. Toy Story Land opened in 2018 and Galaxy's Edge even more recently, Mickey and Minnie's Runaway railway replaced Great Movie Ride so basically the only things that were there were Tower of Terror, Rock and Roller Coaster and the shows. If you just want to see those lands and maybe ride a ride or two, you are good to go just by being in HS, if you want to ride all 3 rides in TSL, both rides that are physically in GE and MMRR, you might want to buy
Genie+ (personally I don't use it but I don't mind standing in lines and don't need to ride a lot of rides).
For a 3 1/2 day trip, with Pandora and HS as the main goals, I'd do the following. Arrival 1/2 day make my park reservation for HS and hit it for at least a few hours, get in a ride or two. Then either walk, boat or sky liner over to Epcot later and hit a few booths for some food and call that supper. Maybe get a snack at HS to tide me over. Epcot and HS will be open until at least 9, maybe later no one knows since they tend to play with ours closer to the actual date.
Day 2, first full day. Make the park reservation for AK, it will close earlier than the other parks and you should be able to get a decent overview of the park until park hopping starts at 2. You will be able to see Pandora and ride both rides there if you want and ride the safari, maybe catch a show, grab some food at Nomad Lounge which is walk up wait list, walk the trails and probably ride EE and Dino. I like to take more time at AK but given that you are even thinking of skipping it, until 2 will do. At 2 hop over to either HS or Epcot, dealers choice. Either one you can knock out a number of rides, if Epcot is your choice you can probably hit most of the heavy hitter rides and snack around the booths and WS.
Day 3, second full day. Make the park reservation for MK, hang out there as long as you like. It really depends on if you want to ride a lot of rides here or just soak up MK, watch a parade, etc. Leave whenever you want and again where you hop is dealers choice. You decide which park you feel like you need more time in.
Day 4, dealer's choice as to where you make your park reservations but go ahead and make them somewhere. You can sleep in or head to a park early and ride a few rides. At this point you have been to all 4 parks. Only you know what park you might think you want to make your reservations for but my guess is HS since you haven't done a full day there yet.
You can hit all 4 parks easy in 3 1/2 days with park hopping, heck depending on where you are staying you can go back to your resort for some pool time as well each day. You don't need ADRs in advance, there are plenty of QS, day of ADRs and other places you can eat at every park. I'm a fan of Nomad Lounge at AK, HBD Lounge at HS, eating at booths at Epcot. MK is a problem but I just grab a snack there and usually go back to my resort before I hop from there. There really isn't a lot of planning that needs to be done and Genie+ or ILL aren't necessary unless your goal is to hit a lot of rides. It can be a nice relaxing trip if you want it to be.
