Your fast prototypes coming by air freight likely aren't routed through USPS at all unless it's the last leg of a consolidated shipment that's broken apart once it reaches the US. Those would be using some other carrier to get them from China to the US and then USPS only inside the US. USPS all the way from China is slow.
Paying ~$30 for express shipping through DHL (plus whatever the new tariffs end up being) will still get you those parts in 3-5 days to most major shipping hubs in the US, your suppliers will just need to start filing the export paperwork correctly.
These changes will likely have bigger impacts on cheap off the shelf parts from e-commerce places like Temu or AliExpress, who were previously taking advantage of both the de minimis rule and inequal international rates through USPS.
Your Chinese suppliers can still ship by any of the normal commercial express shipping carriers as long as they understand how to file export paperwork or have an agent who can do it for them. Previously this usually added 1-2 days to the transit time over shipping undeclared "samples". Last year DHL moved to a paperless system and that extra 1-2 days delay is probably going away anyway. They may have even done it because they saw this coming. People have been grumbling about the de-minimis stuff for a while now.