" For my stocks and non-money assets, I opt not to track them super closely in order to avoid having to update the values of each stock every time. " GnuCash's Price Editor can fetch values automatically. It used to be able to use Yahoo finance, but they cut that off a while back. There is a replacement, but I cannot remember what it is at the moment (and am on the work laptop). GnuCash is actually pretty easy to use…
It's possible to get Yahoo! data through web scraping, but it's certainly less convenient than it used to be.
IEX also has an open API for querying historical data.[1] I don't think it has as much historical data as Yahoo! has, and it's changed a bit since the last time I thought about putting together a backtester.