That is an incredible amount of work. You can see the effort required by google to make Flutter do this - plus it has the benefit of the dart language being simultaneously engineered for the task. Google has spent millions and put their top engineers on the problem and it looks to me to be an uphill struggle. Flutter has 8k open issues and 32k closed issues.
Namecheap I found to be pretty good. I only need basic features so I moved to a cheaper (in Canada) registrar - porkbun and they have been totally fine.
It would be simpler to have something like a google drive / dropbox where you allow access to store things on there to specific apps. Then you allow the data to be retrieved by another app. If the storing app does an export (say in csv format) each time you make changes then the data will always be available for another app to use.
I think requiring an app to use a specific external database probably wont work because app developers have specific needs for each app, perhaps xml or key-value pairs etc. The app is then relying on a third party to provide the performance / latency they need. If a universal database could be chosen I would choose sqlite since that is available on phones.
For the last 7 years I have had a callcentric account. I added telemarketer block to my calls, then callers have to press a number to get through to you which eliminates robocalls. You can assign friends into buckets which get different treatment such as dialing your cell number.
As an alternative to learning Kotlin on Android, you can spend the time learning flutter. Then you get concise code which works on Android and IOS now and on other platforms in the future - including desktops.
These guys have given you excellent market information. The piece of your project with value is the mls integration you are about to write. I think you should continue working on the open source part and keep the mls plugin closed source and sell the plugin for the mls integration to the realtors. imo this plugin will be worth more than $10k and the guys contacting you know that.
There is only 1 reason I stopped using Firefox completely. I cannot search with Google in the search bar. (This is in Linux Mint). I use search a lot and basically all the search engines on offer are sub-par. I use Chromium and Opera also, and have them all running, but I wont touch Firefox.