I think that Hybrid PoS / PoW has some support, but changing to full PoS seems to be too radical a change (in this small user-base)
The smaller the user base is the easier it is to implement. The handful of people who are mining it now can't sustain this coin. Full POS is the simplest solution and now would be the right time to do it before the coin gets any bigger.
At first sight I like your new idea for a variable block reward system that is tied to the exchange rate. But it raises some questions;How exactly would the clients agree on the exchange rate in order to calculate the block-reward and can we allow a block reward value of 0 ?
I would call this a Price Based Rewards System (PBR).
You would collate data from more than one exchange like CoinMarketCap does. So there would be no central authority. You would propagate the value through the blockchain in the same way that solved blocks are agreed on and transmitted. Whoever calculates the value of x (with x being the mean exchange value) first has the value to use for calculation of the next block and so on.
Note: I'm thinking here from a mathematical standpoint. As I've said before my training is basic sciences and not programming, so whilst I understand mathematical and cryptographic principles I don't know how to translate that into computer language or code or indeed how much work it entails. From a human standpoint it is pretty simple but I suspect it is more complicated getting a computer to do it all.
Also you don't need a block reward of 0, 1 might be low enough. I think this is something that would need refinement over time (I don't think testing in private would give enough data) but starting with something like a range of 1-50 would be acceptable.
The more difficult (though not too difficult) issue would be creating a formula to calculate the adjustment. Now this would probably be more easy to program, the hard part is coming up with an algorithm that is fair and effective. This would probably be another situation where actual use in the real world would provide data for further refinement.
The basic methods I was thinking of was to either use an absolute value based system (which would be much easier to implement) vs. a rate of change or gradient based system which would be slightly more complicated requiring a calculation from a certain number of previous values of x.
The absolute system would require more assumptions with for example predicted values for the price of MNR at certain stages and how close the current value of x was to it. This value would also need to be periodically reviewed to account for market conditions etc. The other point to consider would be that the system would be designed to restrict downward pressure on price and not upward pressure, although some mild restriction of upward pressure might reduce the effect of deliberate pumps and this could be added at a later stage.
Like I said solution 2 (PBR) is a lot more ambitious, complicated, difficult to implement, and most importantly unproven when compared to the POS only w. Multipool method.
Note that PBR could also be implemented to calculate interest for POS too.