Couple of observations from my latest campaign.
The Spanish get way too many raider and militia replacements. Those two categorys are always full after the first year and never get used. I have changed the Spanish insurrection event to only give 4 each instead of 8 and they still build up to max too quickly.
Not enough line replacements in the Spanish insurrection event. I doubled them from 2 to 4 and still ran short consistently.
I am adding hvy cav replacements to all campaigns.
I still havent figured out how, when, and why new elements are added. What pre-conditions allow it?
Nor can I for the life of me work out which units can be combined without trying to physicaly combine every unit one at a time.
(very frustrating.) Not sure if the Spanish forces are supposed to have all those tiny units? But it can be pretty daunting trying to organise them into some sort of decent OOB. ( Not nessesarily unfun though. )
I found I had to be very patient and avoid big battles at all cost unless my safe zones were penetrated. Your irregulars and raiders will keep the french busy enough. Hopefully.

Nial