It's actually a pretty crafty country, no matter how unrealistic. As soon as the MC was known for his new riches, he was made a baron, which apparently comes with obligations (probably a certain amount of taxes to be paid to the crown). Since he was capable enough to raise himself to such a position, it's better, in the long term, for the country (the royals) to rather bleed him dry slowly than to rob him of his wealth at once. After all, it's reasonable he would continue to enrich himself and his fiefdom, thus producing taxable wealth for a long time. Furthermore, the clique of ruling women obviously is good at arranging awful marriages (for the husband) that ensure no man will become too powerful.
I suppose if the MC resisted too much, he would need to face the country's military might. So, if he wanted to be totally free, he would likely need to ally himself with a foreign power and play his cards extremely wisely not to get backstabbed and of course not to be defeated. Or he could just disappear and live off the grid entirely. However, since he cares for his family so much, neither choice might be possible. Although a simple solution would be to take his brother with him, maybe even the father if the old man wants a break from slavery. All in all he should become more jaded and less caring.