Sphere looks quite promising, and I haven't come across them before. Their coverage looks pretty good as well, and pricing sounds reasonable. It looks like they have a Stripe integration. Is that something you're using?
I run a B2C SaaS business. We're an EU company. We use Stripe for all billing, and we use Stripe Checkout and Stripe Tax for all of our tax calculations. We've built our own tools to generate monthly/quarterly tax summary reports from our Stripe payments data.
Luckily most of our business is in the EU, so as an EU company, filing our EU VAT is easy.
We used Avalara for 1 year in order to setup VAT registration in Norway. We chose Avalara because they do in fact provide full service solutions for VAT (registration, filing, and remittance). However, they are expensive and were AWFUL to work with: poor communication/support, no integration with Stripe, and we had to manually generate carefully structured Excel files in order to import our sales data. We cancelled our contract with them after 1 year, but were able to take over the filing account from them and now our in-house accountant does the filing and remittance (which is quick, easy, and MUCH cheaper).
We have considered something like Paddle, but it is difficult to justify the increased fees as our business grows. For the amount of money we'd be spending on Paddle in extra fees vs Stripe, we may as well just use a service provider or hire our own staff.
With Stripe Tax we can easily monitor our thresholds in other countries. There are some countries that technically require registration for even a single transaction in that country, which is ridiculous. I imagine for most companies it is definitely not feasible to register, file, and remit in all of those countries.
Started AwardFares (https://awardfares.com) back in 2018. It's a search engine for finding flights to book using your airline miles. Started as a side-project, completely bootstrapped and self-funded.
We hit $2k monthly revenue after about 1.5 years. Growth completely stopped during COVID, but is now growing very well (more than 10x that).
Kind of funny, I'm still on my most recent replacement/repaired unit, and it's still working great since my AppleCare Protection Plan expired (knock on wood). But I guess it's only a matter of time until I'll have to buy a new one.