I am a fan of IndieHackers and one of the members posted an great article about their own tech stack
My own start-up, CryptoStar uses WordPress as the key platform. This is a good choice for a solo developer because of the huge number of high quality and inexpensive open-source plug-ins that can be used to extend its functionality. I use MemberPress for plan management and Stripe for billing.
One difference between the Gym Business described in the article and CryptoStar is the breadth of requirements and the integration philosophy. The Gym Business uses best-of-breed systems with Zapier as an integration platform. CryptoStar is a much smaller enterprise and uses WordPress as the core platform with custom integrations between plug-ins. I am very happy with my design choices but as CryptoStar grows I may need the support of best-of-breed systems and this article makes useful suggestions.