I don't think increasing serial builds is really viable.
I see your point on the surface, a gearing bonus is supposed to reflect a factory getting better at producing something over and over, hence reducing the time it takes and the cost, you should be able to just keep upping the serial order no problem....
However I think theoretically at least, that a gearing bonus is also based in part on orders for parts, labor manifests, resource consumption, transportation needs, etc. If you order 12 panzer divisions in serial, you have to ship armor, and engine parts, and treads etc. That takes time, by the 3rd or 4th division the factory dude as we will call him, he finally has all that stuff sitting there ready for the next 8 divisions. He has his order and finally he is all set to finish it...
But now almighty emperor orders 12 more! What a jerk. This is going to screw up all the factory dude's numbers, inventories and labor projections. What if he now can't get some treads for the 15th division, or he runs out of tank commander cupola covers or whatever. That throws off the gearing bonus right there just from the delay. So you set a serial run, you should be stuck with it.
Now in terms of just setting serial runs of 99, it is a good idea in theory. But...

I am bad at math. If I want to have 166 infantry divisions for Barbarrossa, and I set a bunch of 99 serials of infantry to get there, then I stop them at the wrong time, I end up with irregular corps or army groups! I hate irregularity....plus planning years in advance is easier if you know when a serial is going to end...on a run of 99 something you have no clue when to stop until the next unit deploys...that makes planning years ahead alot harder.