2015 Macbook Pro: can't connect iPad or iPhone

I have a mid 2015 15" MacBook Pro that's been pretty solid for me since new. Two or three times I've had an issue with the USB ports. One time it was while transferring video files from camera cards, and I didn't have AC power connected, which drew too much power from the ports, causing one to stop working completely. The other issue was trying to connect my iPhone so I could back it up, and having the connection cycle on-and-off repeatedly. In the past, resetting the NVRAM or maybe the SMC fixed these issues.

However now it's both USB ports, neither of which will work with either my iPhone or iPad, causing both to connect and disconnect repeatedly. I've tried multiple cables, of course. I've reset the NVRAM and done the SMC reset. I've run Apple Diagnostics, which doesn't show a problem. I've restarted in Safe Mode, but still had the problem.

I have connected USB drives or external HDs to the ports, and those work fine.

So I'm quite stumped, and would welcome any advice or suggestions.

(Obviously I could take the MBP in for repair, I mean advice on what I might do myself.)

Thanks!