okay it turns out the problem wasn't with power at all—I just needed to get the rp2040 to wait a bit longer for the crystal to start oscillating (see e.g. https://forums.raspberrypi.com/search.php?keywords=PICO_XOSC_STARTUP_DELAY_MULTIPLIER) with that fix, it starts up without fail every time, and my SRAM even holds a save! only remaining functional issue is that I can only flash 1MB of my 2MB chip. I suspect this is a software problem on my part (though I only ever used a 512KB chip on my breadboard proto, so even 1MB working is a win)