I'm an agreement with high province to muster as you need some engineering skill, or some recruitment action to get the learned engineer who could then recruit/train the unit for lower level provinces, the regents guide has Artillerists and Engineers to reflect these 'specialists', so the various siege units themselves should just be the grunts.
I would say however that even without the grunts lobbing rocks, engineers should have some benefit in a siege from them saying things like 'that part of the castle is weaker sir, it is less accessible for the defenders to send reinforcements to / has poor visibility/coverage for archers / could be more easily undermined, etc, etc.'
The siege units themselves should only require a supply of manpower and engineering skill, so a few 'active but not besieging' units provide the muscle and the engineers then start building the wall breakers. That approach doesn't work well in a system showing onagers, trebuchets, etc as separate units though - instead you would simply allow each engineer (once they'd stopped in place) to generate 1 or 2 artillery points per war move until they topped out (dependent on skill level, resources) and not have artillery units themselves.
I would, incidentally, suggest that an intrigue/adventure action to sabotage the equipment should be able to delay the inevitable - I don't know about historical parallels but its a staple of fiction. Also of course if the unit moved then its heavy equipment would be left behind.
So you could eliminate the existing units for artillery, and instead say that engineers are deemed to have light siege equipment and parts with them but, so long as they have at least 1 non-elite unite that they can then build whatever else they need on site - I'd suggest a higher than normal maintenance cost to reflect these war machines and attendant costs. I say non-elite as frankly when work involves spades, hammers, etc you need peasants - knights are above such things and lack the basic skills.
I admit to being confused about the difference between artillerists and engineers - I'd suggest rolling these units into 1.
I wonder however if a unit of engineers couldn't improve the odds on a construction action, or 'contribute' 1 GB per season of building power towards it - the latter is slightly fiddly so possibly not worthwhile, but a bonus to the success roll based on unit experience would be fairly easy to track and make it more worthwhile for the larger realms to keep them around.