Okay so I've had some time to play around with this mod and I have to say it is incredibly fun. You can even do dogfights with addon WW2 aircraft like the FW-190, ME-262, P-40E, etc. HOWEVER it does require some additional tweaks and changes to the various vehicles & vehicle weapons in order to work best

Here are some common issues and their solutions:

1: Disappearing textures when add-on aircraft spawn. This is usually caused by liveries. Many aircraft made by SkylineGTRFreak & such have multiple liveries which are either 4096x4096 or 2048x2048 resolution (4K or 2K) using DXT5 compression. This requires large amounts of GPU memory and RAM. I have 10gb of RAM and I run into issues with disappearing textures even with only 3 add-on airplanes spawning with each a 4K livery. However if your PC is sufficiently powerful then you may not experience this issue.

I resolved this by opening the .ytd files in OpenIV for the add-on vehicles/planes I was using and deleting the liveries I don't want. I try to reduce it down to only 1 or maybe 2 liveries per vehicle. Then I export the liveries I kept & use Paint.NET to resize them to either 2048x2048 or 1024x1024. Then I save them using the same .dds format with DXT1 compression & if they originally have mip maps then I generate mip maps using "fant" scaling. (It shows in OpenIV whether or not a texture has mip maps.) I also resize the specular map as well to match the livery resolution.

This can lower the quality of the aircraft's exterior texture a bit especially if it originally had 4K liveries, but it's only noticeable when very close to the plane. If it's an enemy fighter jet 250 - 1000 meters away then you won't be able to notice. I'm personally not bothered by it at all & it makes this script run a lot smoother.

2: Unable to spawn error.

NOTE: Often this error just means the plane can't spawn because the script has already spawned the 3 planes (or however many you have it set to spawn). It doesn't necessarily indicate an issue.

3: Unable to spawn pilot.

Add these lines to the vehicles.meta files for the planes you're wanting to spawn with this script:

Under flags put FLAG_LAW_ENFORCEMENT.

Next replace whichever driver it has with this:

<drivers>
<Item>
<driverName>S_M_M_Marine_02 </driverName>
<npcName> />
</Item>
</drivers>

I can't 100% tell if putting in the driver makes a difference, but this combined with FLAG_LAW_ENFORCEMENT fixes the "unable to spawn pilot" error for me. If there's no driver at all then find the <drivers /> line and replace it with the above.

Also the "Swankness" level seems to have an affect on spawning too. Set it to SWANKNESS_1 or SWANKNESS_2 if it's not already.

4. NPC's have extremely inaccurate homing missiles.

For the vanilla homing missiles you can edit these values in update.rpf>common>ai>weapons.meta.

AMMO_SPACE_ROCKET

LifeTime value="30.000000"
FromVehicleLifeTime value="30.000000"
LaunchSpeed value="1800.000000"

TimeBeforeHoming value="0.100000"
PitchChangeRate value="80.000000"
YawChangeRate value="80.000000"
RollChangeRate value="80.000000"
MaxRollAngleSin value="0.500000"

AMMO_PLANE_ROCKET

(I set the values identical to AMMO_SPACE_ROCKET.)

VEHICLE_WEAPON_SPACE_ROCKET

LockOnRange value="900.000000"
WeaponRange value="1200.000000"

Delete "HomingToggle" under WeaponFlags. This removes the ability to switch between homing & non-homing as sometimes it seems like the AI switches to non-homing and they're terribly inaccurate.

VEHICLE_WEAPON_PLANE_ROCKET

(Again I set these values identical to VEHICLE_WEAPON_SPACE_ROCKET.)

For add-on vehicles with custom weapons you'll need to look in the dlc.rpf each specific add-on vehicle to find the weapons.meta file & edit the weapons there. I also like to increase the WeaponRange value for the various aircraft machine guns & cannons. I normally set it to 500. (The range values are in meters.)

If you want you can also edit these same values in weaponhominglauncher.meta located in update.rpf>dlc_patch>mpchristmas2>ai in order to make the Homing Launcher have faster & more accurate rockets which can travel much farther and change their flight path better. Otherwise it won't be very effective against fast moving fighter jets & such firing at you from long range.

You can play around with these values a little bit to fit what you find to be the most fun. Make the missiles faster or slower, etc.

Lastly, I recommend opening ScriptHookVDotNet.ini in Notepad or Notepad++ and making sure to set "ReloadKey=Insert". This way if you get a script error you can simply press "Insert" to reload your scripts, that way you don't have to close & reopen GTA. It also makes it so you can change whichever airplanes are being spawned without closing the game. Alt-Tab out of GTA, edit CustomWantedAirTraffic.ini, save, go back into GTA, and reload scripts. Then the new airplanes will start spawning instead.

That's all I can think of for now. I know it's a lot of stuff to do but for me it really improves this mod. I very rarely get spawn errors now and it works extremely well.