Divisions having leaders you can directly manage and swap out isn't good, IMO, it's far too much micro. Eliminating divisional commanders and getting rid of corps are two changes in HoI4 that, alone, if applied to HoI3 with no other modifications, would cut down maybe 25% of the micro in the whole game.
One alternative might be to have divisions not just gain experience linearly, but to possibly gain traits too from combat. You would never have to actually manage the leaders directly, but you would still partially replicate the extra flavour and gameplay impact of having division leaders with traits. Your mountain divs that fight in mountains might gain an extra +10% bonus in that terrain for example after fighting in it extensively. Then you'd have ways of selecting "all divisions with X traits in this theatre / army" to help you rearrange and sort them based on your needs for battle plans.
But I'd even live without that, I don't think it's important.
HoI3 did not just have a lot of micro, the micro was poorly distributed. In times of peace there were constant interruptions thanks to the sheer number of techs and the need to constantly manage it. HoI4 has fewer, but more influential techs. In HoI3 you spent time setting up your politicians, but it was front loaded and you only had to think about it once every election (if elections exist). Espionage was front loaded. Fiddling with brigade structure was a huge front loaded micro task that has been replaced by the template system - it's now super easy to alter massive numbers of it and actually distribute the new equipment WITHOUT having to place all the brigades one by one, manually merge them into divisions, and so on. Sorting out leaders and managing the OoB was a huge front loaded task that took FOREVER with large countries. The first few weeks of HoI3 in game time could take an hour of real time for a country like the USSR, then suddenly six months zip by like its nothing, then your techs start finishing, then you have to stop and manage production queues for divisions, then the war starts and you spend tons of time microing combat directly and you're back to snail's pace since you have to concentrate on very low level combat. It really was all over the place like that.
HoI4 from what we've seen should be a much smoother experience, with fewer sudden huge spikes in time consuming micromanagement like HoI3. And I totally respect them for that, even though there are serveral changes that I'm not a fan of and several design decisions I'm questioning strongly.