All these changes only address one side (and the less important one of) of why null mining isn't happening.

Mining as it is, is a very uninteractive activity. You need a bunch of people sitting in a belt for hours, who can do nothing but drag ore from their bay. At the same time you need someone to clear the rats (who gets to do nothing for ~30min, then be active for 2) and if you plan to defend from PvP enemies, you need a sizable gang on standby (which is supremely boring).

The suggestion brought by others to make ore sites (in null only) require scanning is a great one.

Secondly the exhumers (or at least 1-2 of them or a new class of mining ships) need to be redesigned with the idea of nullsec defensive combat. This doesn't just mean lots of tank, because a helpless ship with lots of tank will just get ground down eventually. They need to actually be able to protect themselves, something like bonuses and slots to allow fitting of undersized weapons (or RLMLs) that would allow a pack of exhumers to not be a pushover for a smaller number of inities or a torp bomber drop. For example adding a max 3 fitted to strip miners, and adding turret/launcher highs, more mids and bonusing small weapons on the hull, but reducing warp speed to make them undesirable offensively.

Another idea would be to gate the ore production through availability not time to harvest, much like exploration sites. Make it an intense, high ISK/hr activity where it makes sense for combat and mining players to work together in a group over a short span with high rewards.