It might be worth pulling the seals off the existing bearings to see how much difference it makes? If there the same as the seals that were on my bearings then they do come off with some 'encouragement' and it does appear to make a big difference to the coefficient of rolling resistance. I removed the seals from these bearings in my wind turbine for this reason and covered up the bearing with a piece of steel machined to be a close fit.

I think submerging the bearings in oil is a good idea.