I've had good success with the following card on Mac:
It has Linux support as well according to the link, but I have not tested it. Here are the specifications:
Supports NTSC and PAL
Captures Composite RCA or S-Video input
640 x 480 @30FPS NTSC video capture
720 x 576 @25FPS PAL video capture
Included Software: Videoglide for Mac OS, ...
After some considerations I have decided for HIKVISION platform. I chose that platform over DASHUA mostly because of better support in my country and better firmware upgrade program.
Multiple platform (like DASHUA, HIKVISION, AVTECH etc.) cannot be mixed nowadays mostly because ONVIF G standard is not yet widely implemented. Some attempts like Ozeki SDK ...
From a cost perspective, your best bet is something used off eBay. If you want something new, it will likely be way overkill. Hauppauge has always had solid capture devices, and the HVR 1955 appears to have Linux support if you get the right one, as do many of their products. This one happens to support HD as well (I doubt you'll find one with Linux ...
The Elgato 4K60S+ seems to check all of your boxes:
4K resolution @ 60fps
Portable and Standalone
SD Card slot
It also has HDR10 capabilities.
I've had success using two devices. One which is a self-contained device:
Composite, S-Video, and HDMI to HDMI Converter and Switch with HDMI PAL NTSC Support - Monoprice.com
and the other which is a video capture card:
Diamond MM VC500 Video Capture Card
240p — HD Retrovision
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