1. Merchants of record can make sense for smaller B2C companies with lots of international revenue that don't want to handle taxes or chargebacks/fraud. But the fees are extremely high and often your company would only be remitting in a few jurisdictions if you were handling this independently. Respectfully, a lot of the MORs are selling fear more than something you actually need for a lot of sellers. If you're already on Stripe, I'd stay there vs going to a Paddle. Prob not worth the fees. StripeTax is good but very expensive and you'll need something else for filing, registration remittance. A bunch of new startups have hit the space in the last couple years (several of which have been mentioned here) that can help with US and/or international calcs, filings, remittance.
Just do your diligence and don't get sold something that's overkill for what your business needs.
2. On that point, not legal advice but from a risk management POV, below a certain amount of revenue ($500k-1M in ARR), it's probably fine to temporarily punt on these taxes or to just collect & remit in your home state / country. You probably won't have nexus in most jurisdictions early on anyway. Additionally, many VAT countries don't tax B2B software so if you primarily sell remotely to other businesses (sounds like you are mostly b2c so not relevant to you) you might not need to worry about VAT in most countries, just US sales tax. Additionally, if you're based in the US, you're probably at higher risk of enforcement here than you are abroad. Washington or New York is more likely to catch up with you than a country in the EU (or vice versa if you're based internationaly). tl;dr - get PMF. And then handle your home country / states first. Don't freak out about this at a lower volume of revenue.
Full disclosure I'm the co-founder of Taxwire.com and we're a startup in this space. We do registrations, filings, remittance, and calculations in Stripe, QBO, and other billing/payment platforms for the US and recently launched VAT as well. So obviously I have some bias on solutions but hopefully the above is helpful.