How about since we are now at the end of the tier rewards.....?

91 missions completed = 2nd item of 3 mission
96 = 2nd of tier 2
101 = 2nd of tier 3
105 = 2nd of tier 4
111 = 2nd of tier 5
116 = 2nd of tier 6
122 = 2nd of tier 7
ETC.

When 'round 2' is finished, follow the reward system and do a 3rd item of x tier. Note that this follows the +1 more completion required every 6 rewards currently in place.

The problem with this is that some tier rewards are unlocks (such as the soldier outpost and omnibase) and therefore cannot repeat.

NP make them unlock soldier outpost MK2 :>

I would say they would all need to be a version 2 of the item, upgraded bigger and better, because the items would seem weak for earning such a high level.

That does make sense, but some things would get too powerful. After all, going around again would allow some players to get practically zero seconds per point of energy...shields too. That would make them too strong. It would just be better for Webguydan to make a ton of tier rewards beyond the krionite transducer.