The 3.5 SRD would be a great stop-gap and certainly gets my vote but in this case I think it's down to the numbers ... how many folk are going to buy something that's quite out of date now? In terms of 3.5 specifically I've ended-up inputting almost all the 3.5 rules manually in the form of handouts with tons of links to reference other pages. It does work really well but no way I'm going to recommend you spend several hundred hours doing that! I see you have a PRO subscription. Not sure if you're aware, but you can now import PDFs so you could do that to help with rules references. FYI Paizo tried releasing Pathfinder 1e on Roll20 and they abandoned the project due to lack of interest (i.e. sales). There are some elements you can buy so if that's good enough for you, you could go that route? For example, if you purchase Rise of the Runelords Anniversary Edition you get the Pathfinder 1e SRD. In addition, the new Kingmaker Adventure Path by Paizo has been released with extra modules you can buy for both the Pathfinder 1e and DnD 5E compendiums, so perhaps we'll start to see more of these multi-system releases in future?
