It is not the cheapest of the options but i would recommend getting a box blade, as mentioned already. It will serve the purpose of rear weight, let you do box blade stuff as desired, and has basically a 100% resale value. I suspect if you only do spacers but no 3pt weight and no tire fill, you will end up doing spacers AND one of the other options. At that point the question is, are the spacers worth bothering with? There's no amount of wheel spacer that is more effective than lowering the bucket the same number of inches, and if the bucket gets close to dragging the ground and it still seems likely to tip, i would say whatever you are doing is unwise and tinkering around the edges of the tractor design will likely not change that.
So i would just carry the bucket lower when possible, drive thoughtfully, and buy a box blade and do fun box blade stuff. My .02
there’s no reason to not take a multi faceted approach to solving the problem. To that, a box blade doesn’t have enough weight generally to maximize the capability of the loader. The manual states to use 600 lbs, and many people use more than that, a correctly sized box blade is not going to come close to that.
Wheel spacers, rear weight and/or filled tires is the right approach to solve the OPs problem, and given that he plans to use it primarily for mowing, spacers, removing the loader and some smaller amount of rear weight may be the best option as it will minimize ground compaction will giving him the best stability.
In this case, either a ballast box or better yet a weight bracket, as the OP can easily raise or lower how much weight he is carrying on the rear if he is mowing versus using the loader. Suitcase weights are easy to take on and off to fit your application, but you certainly pay extra for that convenience.
The B2601 has an OEM option for 1-3/8 wheel spacers (when not using a BH), so clearly Kubota thinks there is some level of improvement to be had for hill stability.