I hate brace with a passion, just throwing it out there because the problem is in desperate need of a solution. Maybe a different, less cancerous variant of brace that only protects against indirect damage with 3 minute cooldown and and you can't fire during the cooldown?
What about the Priest? It packs all the punch of an ML20 but cannot effectively be taken out by offmaps (against competent players anyway). You can only hope to catch it out of position with a tank or Schrecks. On the maps you mentioned, you wouldn't be able to take out a Priest, which makes me wonder if this would really be as overpowered as you say. But I would take any solution to the current problem, because I really think functional tube artillery is the missing piece in the equation of CoH2 gameplay.
Still wouldn't want brace on any artillery weapons. They have to fight to get to your artillery gun and either A. kill it (not just decrew) with tanks or B. bomb it with off map artillery of which for ost is stuka strike or Railway and OKW some very expensive off maps which is hard enough to achieve. Give it brace and its like having an invincible mortar or mg that can damage enemies without being destroyed.
Priest and calliope are way harder to kill than static artillery guns due to mobility armor and HP but they also cost a fair bit of fuel as opposed to just manpower. Priest IMO is balanced. Good vs static targets (namely OKW trucks and pak43s) and great survivability. Calliope, while being more expensive pays for itself without even seeing an enemy. Thats not OK IMO.
Artillery should be a SUPPORTING part of COH2 imo. Artillery is LAZY, ANNOYING and FRUSTRATING to deal with, and makes discourages proper assaults and strategies. I remember COH1 games where over half the game was about dodging enemy artillery pieces while trying to build up a strong enough force to defeat them. I do NOT want that again. I don't think any competitive players or relic want that either. Thats my opinion though. At the end of the day whatever makes relic more money will likely be the direction COH2 drifts to.