Frisia is at the edge of the map so noone bother them and they can have good mid tier infantry and cavalry faster than others, though they lack of elite troops.
if you want recruit men in villages you need the permission of the lord owner of the village, to get the permission you need to have at least +5 relation with the lord.
Among other things, who tell us that we will have enough berserks to create an effective fighting group as cavalry, archers and infantry, maybe it will be difficult to have more than 5/6.
In this case it would be better to put them in the shield wall and let them do their work until the formation is disassembled by charging the enemy.
It depends on how you use them, i use the cavalry to chase the runaway enemies and the archers and skirmishers to weaken the enemy frontline before the charge.
The two-handed berserkers will make the archers more useful because no berserker will reach the enemy shield-wall, unless you want to use them to charge the back of the enemy.
This would make them useless especially with two-handed axe, charging a shieldwall without shield (often also with) when the enemy are throwing javelins at you is certain death, believe me i tried it.
If it work as in Brytenwalda if the enemy try to break the encirclement and they lose some troops this means more food for the survivors in the castle. The days of food are recalculated according to the new number of soldier in the garrison.... or at least I think it works that way.