With banking / brokerage platforms like Alpaca, it's possible to create an open source roboadvisor, but I'm not sure who the market would be. Someone who is interested in algorithmic trading would go directly access the API's, and someone who wants a hands off experience could choose from existing products, or get bundled services from a big bank.
I'm squarely in the frankenstein of spreadsheets, but also made a mobile frontend in https://jch.app The people I've talked to who use spreadsheets do it because it's fun.