Recommendation for converter from MiniDV and Hi8 to a Mac?

I doubt very much that there'd be a noticable loss in quality dubbing the hi-8 to dv and you'll then have a source with timecode that you'd be able to recapture with frame accuracy if necessary down the road.  

As previously mentioned, a Firewire to Thunderbolt converter cable (available at an Apple Store or online) should work with connecting to your Sony DCR-TRV900. However, the camera has the small Firewire port (called a 4-pin) and the Apple adapter has a square plug (8-pin or FW800), so you'll also need a converter cable; fortunately those are still available online and cheap. Once it's all plugged in, Premiere should recognize the camera when you initiate a video capture.

As for the Hi-8, I would normally recommend that you use the DV camera as the deck to pass the analog signal through –– but, after reading it on the TRV900 it sounds like it does not allow "pass through" from analog to the Firewire output. So if you can find an older analog-to-digital converter box that will work, otherwise you'll need to undertake the tedious task of recording all your Hi-8 tapes onto DV tapes (hooking the two cameras together) and then capturing from the TRV900.

*NOTE* -- I hope you are not on MacOS Catalina. Apple has removed the DV capture ability from the latest OS so your camera would not be recognized as a capture device in Premiere.

HTH,

JVK

I think the DCR-TRV900 has a firewire output.  On the mac you can get a firewire to thunderbolt converter (if you're computer doesn't have a firewire connection) and you should be able to capture directly into premiere.   I know people have had some issues with capturing in recent releases, but worth a shot.  You haven't said what OS you're working with, but I think quicktime player has (or had) some ability to capture via firewire.  

I think it is no longer sold but you might find one on ebay - Grass Valley ADVC 110 external digital converter