What you said is correct, all the modifiers and unit cost affects the active upgrades, while the passive upgrades is only affected by current strength, which is the only parameter that modifies the 720 number. I tested this in my game, putting 0 ICs into upgrades, and pretty much most of my units gets passive upgraded all at the same time.That's certainly true for active upgrades in which you pour IC.
But are you sure that this is also the case for passive upgrades?
Correct for active upgrades but I think it doesn't apply to passive upgrades. That's the reason why it is usually not a good idea to try to upgrade a 1916 infantry to a 1936 infantry via passive upgrades. My understanding is that it would take a minium of 10 years (or 30 years if the unit is only at 1/3 strength), 2 years for each single upgrade. Better to do the early fast and cheap upgrades by spending IC via the upgrade slider and the last, usually rather expensive upgrade, via passive upgrade. A cheesy workaround, though, is to passively cross-upgrade the oldest unit types eg 1916 inf to 1935 mountain (or MOT) which is counted as only 1 upgrade (and an horribly overpowered feature).
But all above are not 100% sure statements. I have never really bothered to test and check it by writing down the times.
What we are quite sure about after discussing it here, what certainly applies to passive upgrades:
# strength of the unit is a factor (see above posts)
# and each 1% reinforcement raises also upgrades by 0.5% (see misc.txt line and I have also checked it)
And my guess and feeling is that everything else is fixed around the 720 days for passive upgrades.
So yes the best strategy is to prioritize active upgrade all your units to about 1 or 2 generations older than the latest, then switch to passive upgrades. The best use of passive upgrades is to mass upgrade to different unit types, because those are always too expensive to active upgrade. This is why this is so OP for countries like China who has a ton of militia and infantry and not much else. I tried using this, and after the few years I converted to a ton of MEC, MOT, CAV, MAR, MTN units, you can easily have a more advanced army than the likes of USA or Germany.
I do wish there are better controls for upgrades though, it's very tedious when you have a ton of units. And it can be so much micromanagement, it's too much work to minimize and direct the ICs to upgrade only the desired units.