I've tried this with Google Sheets add-ons. It can be hard since there is no built in payment method so I end up using lots of API calls to Stripe to check for current subscription status. However, you have to get the user to subscribe on one site and install the add-on on another.

Yes, about half my code and effort is handling stuff that an "app store" usually would.

Things like this put the "a 30% cut is outrageous!" into a different perspective for me.

It is still outrageous because those two big ones make a huge economy of scale. They did not develop a payment method for a single app but for millions.

Services are frequently priced by value to the buyer more than by cost to provide the service; I don't see a problem with that.

I think what's more outrageous is that more devs don't find it worth to do a website yourself and get a 10x cost reduction on transaction fees

Which is even better now that it’s 15% for small business

I don't know much about this ecosystem but do you see an opportunity for a business to supply these tools (or be the app store) for excel/google sheets addons?

There definitely is for excel. Both for consumer add-ins and internal business add-ins. I've thought about it, and even started on the idea a few times, but have given up. There are just so many components of the system that are outside of your control as a developer. Someone with the appropriate resources might be able to do something neat.

I thought about developing a simple Google Sheets add-on but I've been put off by Google verification process because of the scopes I use.

I've thought about selling my code snippet for handling payments on something like Gumroad or similar. However, it's not a great solution since people still need to sign up somewhere and then install add-on. Plus they need to subscribe on stripe with their same email that the add-on will use (since the code grabs their email programmatically). As I'm thinking through this, there could be ways to mitigate some of these issues but who knows!

There is someone that built something similar but for browser extensions (https://extensionpay.com/).