I like the supply system as it introduces a good dose of realism
When I reached Moscow with my 15 motorised brigades (the armor was 300km back having run out of fuel) they were still at full strength and full org.
Thinking back to the first German units to approach Moscow IRL - well I think one only had 12 Panzers left and they were all but dead due to the reinforcement lag and the much heavier fighting than I experienced. One of the reasons they got pushed back quickly.
Now from my understanding a division on the move will take strength losses whether it is in combat or not. I would suggest that we need something like a 1% a day strength loss for any moving brigade (with possibly a new tech to capture forward engineering units to repair broken trucks etc) - especially if they are hard - possibly 2-5% for them - modified by the reliability tech.
This would have a number of effects on the game. It would limit offensives to a certain point. IRL they alays run out of steam as mean and material breakdown and get exhausted and you need a pause to consolidate. It would also push people into using strategic movement more - and if you gave some HUGE penalties for units in strategic movement entering combat then you have some wonderful oportunities in breakthroughs for chewing up unprepared divisions.
So just as the supply system got 'fixed' so to should the reinforcement system.