My understanding is pool aiding wasn't supposed to kill cowing, it was supposed to balance it. The goal should be to make multiple strategies viable so that you have to choose between them, unlike the previous scenario where you always wanted to use pool on your biggest and unlike the current one where you always want to use pool on yourself.
Therefore my suggestions are several.
1. The tax should be variable depending on province size. For provinces at the median or above, the tax should be the maximum (call it T, currently 75%). For provinces at half the median, it should be 0. Make it linear in between. For provinces under half the median, there should be no tax. This incentivizes exploring smallies since you can aid them with no pool loss and it's cheap. And note that doubling a province from 50% the median up to the median size is going to raise the median and could still be used to make cows, hence the sliding scale. At half the median size you just end up making another median province and no tax is fine. Since you generally want t/ms around median size or above as well, that will be taxed also.
2. Make T 0.5 instead of 0.75. This makes exploring large provinces viable.
3. Aiding should be done from the explore page. There needs to be a slightly more complicated mechanic then that warns you what the tax level is for the exact province you have chosen. A confirmation dialog would be highly recommended. This is true even in the current aiding mechanism.
The value of T and the exact cutoffs for 0 and T tax could be tweaked of course, but the sliding scale system makes a lot of sense to me. It should be re-assessed each age and tweaked regardless to ultimately achieve balance between cow use and core use.
Note that a simpler system would almost serve the same purpose. You could have a kingdom pool and have explored acres count double for provinces above the median and normal for provinces under half the median, and the effect is the same. Except then there's no kingdom cooperation required and one province can steal all the pool, which is probably undesirable. So I actually do like the current system.