I thought the same splitting into groups would work. Somehow while testing a bit i noticed that the loading function is bugged in game. If you select for example 10 t1 arti and 3 t2
transport planes then press u and click somewhere -> one
transport plane is loading and despite an unload order does not move anymore, two planes simply fly away emty and units not loaded simply run to the place where they should be loaded and create a blob of units bumping agaisnt each other. It shows that the game atm cant split the units correct. This command does not work when more than one plane is selected .... Problem appears because there are no "load" and unload buttons. It is the same button. In fact it is only an unload button which includes somehow loading partly but not correct. You can right click a plane with a unit or a unit with a plane but there is no button command in the game for this like -> select 1 unit and 1 plane -> press 1 button and the unit will be loaded.
Since in the beta ferry is replaced with a marker like function would be great if you can add the ferry "marker" next to the markers and use the free button slot in the units for introducing a load button which clearly is for loading units only and is able to split them if you selected several transports and land units. To spread them equally thats already luxury but even the basic function is not in the game
An additional advantage is that you can even while not having a trans plane create ferry routes while beeing idle.