This is an interesting one. If I’m understanding right, you want to remove the auto-promotion between trust levels and set your own criteria?
If so, I think you’d need to set a couple of the TL settings to something unachievable. For instance, TL2 requires days visited to the max of 36500, and TL3 requires days visited to 101 (based on TL3 time period being the default ‘100’). This would then prevent any user from automatically being promoted any higher than TL1
I think then you could enable the hidden site setting (Enable Badge SQL) and use custom SQL badges with some new criteria based on gamification scores, or whatever you fancy.
If you then created some new groups which automatically granted a trust level to members, you could then use these along with a couple of ‘User Group Membership through Badge’ Automation scripts to award a badge when they hit the criteria, trigger the automation, and then get included in the group which grants them the required trust level.
I think some bits of that will need refining/experimenting with, but I think it’s doable. 