Imho, just go P2P subscription based, period.
Probably somewhere between 8 ~ 13€ monthly would be accessible for everyone.
For the Kickstarter packs (and future packs to come), give them X monthly access depending on each pack and the perks already included.
25€ = All Rewards
50€ = All Rewards + 1 Month
100€ = All Rewards + 2 Months
Since 2 Months it's too "little" for the 100€ considering you are paying 100€ (which you are getting worth from Oath Tokens + Kickstarter Exclusive Perks + CB Acess, etc.), ReadyUpStudios can focus on decreasing the Oath Tokens in each pack but gradually increasing the Months game-time, resulting in:
25€ = All Rewards (1250 750 Oath Tokens) + 1 Month
50€ = All Rewards (2500 1250 Oath Tokens) + 3 Months
100€ = All Rewards (5000 2500 Oath Tokens) + 5 Months
... (I'm too tired to do the proper maths, besides the fact we still don't really know how much worth 1 Oath Token is in real-life currency, but I hope you get the point of what I'm trying to say).
Again, this is just an opinion and what I suggested is only one way to approach it in case ReadyUpStudios considers changing their game model from F2P to P2P subscription based.
To be honest with you, I think that the Cosmetic Shop is and will NOT be enough to keep the game alive.
I trully hope I am wrong but regardless of what happens, I hope they are able to switch back (one way or another) before it's too late.