Plan in detail for MVP (Minimal Viable Product). When you are bootstrapping and you will be outsourcing the development, it is best to do it in iterations.
When you look for an agency/developer/s see their portfolio, identify previous customers and contact them for feedback.
Best to have it through referrals. I am 15y+ in the industry, and when someone asks me to refer a developer or an agency, I always tell risks, as all people have strong and week sides. I usually prefer not to refer people, unless I am very positive about the outcome, as I will have to put my name to the connection, and follow up the execution.
When you have a contract for the development, make sure you have a long term support clause. Normally it is an % of the base software value paid annually. You should have this at least for 1-2 years for each feature you produce to keep the product stable. So make sure you have this in your budget!
I suggest you talk to a professional, before stepping into long-term relationships. You need to know what you will be producing, how are you going to distribute it. Do you really need an Android/iOS app. E.g. we have a mobile app SaaS business, and after having apps for Android/iOS we decided to go with a PWA app technology, and just dropped the native app support, as it was much faster to install, as our users were coming from our website. But if you rely on the App-Store listings, that's another story, bust still you can do a hybrid app. It is a decision that might increase your costs drastically if you want to pivot once the product is completed.
I advise you to do all technical specifications in detail, including the mock-ups first. Then do the design, then proceed with development.
This can be done either by the agency you hire, or in advance by an advisor/professional in the field, and based on that find the best agency.
Never start development, before it's all figured out in advance!