I think, that amount of sandbag per base should be limited, because huge sandbag spam makes stealth infiltration impossible.
There was a lot of matches, where both teams had a lot of players and no-lazy utility guys and the only way to win - was win by tickets in SD. That's kind of boring.
So, I suggest to make sandbag limit near you base: you can build sandbag under the limit if you place it within some range (like range for turrets, for example) and you can spam sandbags unlimitedly when you are out of range, far from your base.
Players also can spam steps and platforms, so that stuff should be limited in the same way.
For example, per base:
- 10 instances of sandbags (2 per buildin),
- 20 instances of 1-step-up (3-step-up/forward costs 3 instances),
- 3 instances of platforms

By saying "huge sandbag spam," I assume you mean placement of sandbags that cover a big area, and not just the "spam" where you only cover most of your doors with sandbags.
In that case, I got good news for you - lasers can't be blocked because they pass right through your target and can hurt anything behind your target. That makes it a lot easier for stealths to clean them up. Just position yourself and aim in a way that your lasers pierce through as many sandbags as possible, that way you will destroy more sandbags at the same time.

Or team up with a guardian mech who will fire a nuke straight into the pile of sandbags (or you can be the one piloting the guardian with the stealth class, just eject afterwards and infiltrate).

Also, explosions do radius damage (which means explosions can't be blocked, thus having the same effect as lasers), so there are other useful weapons for cleaning up sandbags: artillery cannon, remote c4, weapons from mechs, teamwork
And that makes the strategy of putting multiple sandbags in the exact same position effective only against bullets, flames, shock rifle, and some weapons I forgot to mention.