It might be worth offering two pricing options: (1) Pay for private answers, (2) free/bounty open source answers.
Your concept seems to have some overlap with StackOverflow. A problem with StackOverflow is their content is licensed CC-BY-SA. That can make some answers dicey for companies to use. Publicly visible answers on your site might be more valuable than StackOverflow answers if you released them as CC0. CC0 answers could also be merged into the docs of the FOSS projects. Having those answers on your site should drive more search traffic to you, bringing more leads for paid private Q&A.
On the expert side of what you’re doing... if you can streamline the payment process & generate a 1099 for taxes (like Lyft-style companies do) that might remove some overhead for experts & get them interested.