Many options will be free if you are cool with their brand logo there are tons of options. Mailerlite seems good too. I tried sendinblue then deleted the account because of cumbersome onboarding for startup at least for us wanted us to prove to them how we get emails and not spam.
If you're comfortable with technical stuff, I would say Make + Gmail Pro account. It will cost you $20 per month for unlimited users.
Otherwise, Mailerlite is the best I saw so far :)
Mailshake (https://mailshake.com/pricing/) might be useful and it's not so expensive. Also it would not hurt to ask for startup credits, many of the tools grant them if your scale is not so large.
CypherChat