Resetting cost basis after moving to PR

The market has dipped and I have US-sourced assets that have taken a hit but still have capital gains. Can I sell and buyback immediately to lock in lower gains (taxed at standard US rates), while establishing a low tax basis for Act 60 treatment of newly PR-sourced assets? Do I need to wait for 30-60 days before re-buying in accordance with wash trading rules?

Yes, you can. Wash trading rules only apply to capital losses. All gains on those assets from then on would be PR-sourced assuming you are still a BFR-PR when you sell them. You can read up more on how losses and rollovers are handled as a BFR-PR here

Additionally you should elect to bifurcate if their value was even lower on the date when you became a BFR-PR. IRS publication 570 rules on bifurcation for marketable securities says you’d pay US capital gains on the fair market value of the assets on the day before you become a PR BFR.