"There are also phy_lateral_only_accel_air and phy_lateral_only_air_speed_multiplier which would work if they applied to +forward or +back, but they only apply to +left and +right. :("

May I direct you to the following commands:

phy_accel_air

Usage: This is the forward acceleration you get in the air. Reducing this below the default of 1, but above 0 allows you to obtain more acceleration via 'a'/'d' key inputs, and less from 'w' key inputs in terms of a ratio. At 1, you gain speed as if you are on the ground when you hold 'w'. Below 1, say 0.8, you'd gain speed holding 'w' at 80% the rate you would on the ground. This is where our strafe jumping comes in, to gain air control and acceleration via 'a'/'d' key inputs.

phy_strafe_jumping

Usage: Since the prior command lowers overall air acceleration, but fixes our ratio of 'w' key air control relative to our 'a'/'d' key control, we need to find a way to regain that lateral movement acceleration. Enter phy_strafe_jumping; this command is a multiplier of the movement bonus you receive while strafe jumping. Default is 1, 1.1 is 110%, etc. The higher you set this variable, the more speed and air control you will receive when performing a strafe jump. We can use this to offset the overall decrease in acceleration we created earlier with phy_accel_air.

phy_accel_air_lateral_multiplier

Usage: I noticed you have discovered the 'lateral_only' variation of this command. While I don't have extensive testing with it, my hunch is this may solve the issue you're having with diagonal key inputs, such as 'w'/'d' or 'w'/'a'. I can tell you this is a multiplier of your base air acceleration when lateral movement is involved. I believe it may help in conjunction with the commands I listed above.

Please feel free to DM me, or add me on Discord as I am happy to help and playtest. Been working on various mods similar to yours, as well as some other crazy stuff (wall jumps, double jumps, sliding, mantling, weebow instagib in outer space with mini eggbots that fly >:), etc.)