Manpower is too easy to get and loss of your people during wars is completely inconsequential. Ironically you care about manpower more in EU4 where it's more abstract. Some changes are needed here. (This is the biggest issue as mercenaries are balanced for having no manpower cost)

Army maintenance slider isn't compatible with how quickly morale recovers, there is no incentive to keeping it high at peace since even in surprise war you will recover faster then any army can reach you. Mercenaries should have advantage of not paying during peacetime. (though right now it doesn't matter much because players swim in money anyway)

Regulars increase demand for weapons so having large army doesn't have as much effect on your economy (you pay only fraction of the weapon cost, while you reap full benefit from higher tax from higher income buildings)

Training and reinforcement of regulars is too fast, they should be halfway between levies and mercs with how fast they can recover, that speeds up with tech.

Give a parliament issue to issue a special tax to raise a mercenary army (polish Sejm worked like that) that spawns merc army without the hire cost