WRT timebase correctors, a lot of the standalone models have SDI output, so you can save some money and a generation of D/A and A/D conversion by just getting a suitable SDI interface rather than a capture device, as long as you have the drive capacity to handle uncompressed capture. I’ve ben doing all of my capture from tape through a For.a FA-370 TBC that was something like $40 in like-new condition a few years back and don’t even use the old Aja card I got when I started doing VHS archiving because it has to pass through the TBC either way. If I’d known in advance I could have gotten set up for about 1/3 what it cost.
For upscaling, some of the line doublers aimed at the retrogaming market are an option that could be worth exploring. OSSC is the most well known but it doesn’t accept composite or S-video. One of the RetroTink variants could be an option, though. Since they’re aimed at people who are trying to upscale video from problematic, off-standard sources with minimal artifacts they seem like a natural for the sort of stuff we’re doing.
EDIT: for downscaling (to use HD sources with SD equipment) the best price/performance I’ve found are the AverKey300 series VGA-to-composite/s-video boxes that were aimed at offices looking to hook up laptops to SD projectors or TV sets for presentations. They’re really common, quality is fine if it’s gong to be mangled anyway, and you can get them for $5-$10 complete in box if you wait around for the right price (or you could two years ago).