Hi
Sir answering your questions first for financial/tax software.
1. Searching customers like putting a keyword "financial help" or "financial", to bring you up in the google, is a job related to SEO professionals. Who will make your site to bring up in the google while searching for the respective keywords.
2. Second way is via google ads, admob integration to display your ads into your related functionality.
3. It can also be with social media marketing.
All above points except point 2, are the tasks needed to be done by a SEO professional and Marketer, not a developer.
For developing the program, i can use, PHP, Bootstap/HTML, Wordpress as this relates to you.
I can share more details with you on chat.
Waiting to listen from you
Regards