I second Tom's response above and recommend that you find a technical co-founder (this is something you can do with a deck and interest based sign ups) and build a MVP (minimum viable product) by bootstrapping and then raise the capital. This would test your idea at no external risk. I've been where you're at and successfully navigated my way around. I've personally raised Family & Friends money several times and will tell you that it's your relationship and reputation that's riding on that investment so be very careful with that raise and setting expectations.

I did sense that you're not confident about your idea's success or at least you didm;t sound like when you said - "Since I've been down that road before I know it might be easier to raise now, with a much lower valuation of course, than later, when the product's ready and then I'll be expected to show traction before raising." Well yes, so are you not sure that when the product is ready there wouldn't be enough users/ traction? And if not, what's the point of developing something you're not confident about? Also, 20K will only go so far so you will have to raise several rounds even after the product is developed with original 20K and the same question of traction will be raised anyhow.

Happy to help with ideas on bootstrapping, finding co-founders, outsourcing the development, or any follow up questions you may have, etc over a call.