How many messages are you expecting to send? 300 - 500 USD/mo suggests hundreds of thousands of messages. Are you expecting thousands of users? If so, any chance that some of your users will pay for your service?
But when you start sending sms internationally, the costs can explode.
As memory serves, twilio charges about 10c to send an sms to Germany, for example.
I run a bootstrapped and somewhat successful monitoring saas and we literally keep a running balance of every users sms cost (as reported directly by twilio) and every user has a $ cap that resets monthly.
By way of example, my company is sending ~1.5M segments per month at a cost of a couple tens of thousands (I can't say exactly, obviously). This boils down to the fees that I mentioned, international examples that you gave, as well as the differences between SMS and MMS messaging where it's not always clear what counts as MMS (multi-byte encoding, anything with an attachment or URL, >N segments per message where N changes based on delivering carrier, etc.) It gets... Wonky.
Did you evaluate going all in on Toll Free texting perchance?