Bastion mode giving scanning probe immunity would be so amazing for making them workable [...] low/null mission runners
Alas I will be ignored I think
I think that'd transcend "workable" and straight into "frustratingly immune", especially for missions although also for complexes. Aside from the hull itself, what can you probe in missions sites - drones, and wrecks (I think). Both have considerably smaller signatures than the ship, giving the pilot plenty of time to run away to the safety of a cloak/safespot/POS.
Now, a scan probe reduction in effectiveness - carefully balanced to give some benefit but not effective immunity - would be better.
Missions can only be probed through a ship or drone; wrecks are not scannable. Complex sites (unless escalations) can be scanned down through the site itself.
I generally don't envision Bastion mode being used for sniping (150km+) as long as on-grid probing is a thing that exists. You will be scanned down and warped to immediately, while stuck in a fixed position ... You might as well set yourself for close to mid range and fuck all notion of long range fire support. This hurts the Golem in particular the most, as it could easily do 200km~ sniping with Cruise missiles. On the flip side, it beast modes with 50-70km Torpedoes and an absurd tank.
:V
Being immune to probing for this in missions helps to stop the disadvantage Low/Null mission runners have to contend with in their territory. You have two means of protecting your mission: leave it entirely and wait, or "Get more bros and out number them/their hotdrop/their batphone/etc". If you are not in a station system you must bounce constantly, sit in a pos, carry a cloak, etc, while you wait for the hunter to leave.
Someone cruising through looking for pickings can easily interrupt your work and you must disproportionately respond. Tech 3 Cruisers and their probe immunity gave an option in this regard, but one that is even what I think is too powerful. A Marauder doesn't have a CovOps cloak or gang links, and they're much easier to catch on gates than a T3 is. At least in this respect, your option is now "Well I'm in the last room with Bastion mode, I can finish the mission and then run/hide/log off".