Look at Bregman’s very bad fielding numbers as a SS, which are also 5 years old. He doesn’t have enough of a sample at 2B, but his middle infield performance shouldn’t be expected to improve now that he’s past 30. Also if he has the arm for 3B (what’s this about bone chips in his throwing elbow?) it stands to reason his range, not throwing was the issue at SS.
For Defensive Runs Saves, Bregman's rankings at SS by year are 0, -1, 0, and +3. The last year he played there was 2019. His DRS rankings at 3B are +2, +8, 0, +3, -5, +5, +6.
Savant has his numbers at SS as 0, -1, -3, and -2 Outs Above Average. His numbers at 3B are +2, +5, +2, +2, +1, 0, +8, +2, and +6.
I think defensive metrics like these are pretty rudimentary, but the trend is consistent that Bregman ranks much better as a 3B than SS. Based on this comparison, I would have very little confidence that he would be a great or very good defensive second baseman from age 31 forward.