Consultation for full stack tech rewrite with mobile apps.

I have several websites all doing the same thing (dating website type infrastructure), but targeting different countries. The tech is old (zend/mariadb) and I would like a complete re dev for with mobile app (android and ios). I would like to consider all the ways this can be done to make the process as easy as possible. Things to consider are : Maintaining the existing and new databases and tying then together during switch over. I would like to use either react native or flutter for a hybrid mobile solution. But the question is how to add the web element to keep the ongoing dev to a minimum. Admin will all via the web so the app only need client facing functions. During the redo I would like some medium size upgrades to function. I believe there will be plug-in solutions for these new functions, which are: i) distance from user member searches. ii) registration with fb, google apis etc. iii) automated picture filtering for nudes. iv) The removal of user profile texts with predefined/selectable profile descriptors. The automated selection of predefined descriptors base don current profile texts. Basically just word matching. I would like to keep the multiple websites operating so would like to look at options for allowing the use of one app with multiple sites and databases. I realise that merging everything to one dbase might be an option but I would lose seo value. So I need a consultation with a company that has good experience is this kind of mass code rewrite to talk through the pros and cons of different options and to look for the best value solutions. This is a consultation to examine to possibilities, there will be no discussion of doing the actual work. You will need at least 50 projects completed on freelancer to be considered.