Ok, using your registrar to serve your DNS records for you is a choice on your part, and if so, then yes you would be subject to their limitations. But the vast majority of registrars, and this includes registro.br as far as I can tell according to a machine translation from Portuguese to English [1], allow you to delegate a domain to your own nameservers, and this is the standard way to use a registrar. You pay a registrar to lease a domain, not to host it. Once they have delegated to your nameservers via NS records, you can serve whatever other records you want (subject to the registrar's legal policies). Now, maybe a lot of client software commonly used in Brazil does not support multi-string TXT records, and that would still be an issue, but it's not on the server side.

[1]: In the 5th bullet under section 1.2 of https://registro.br/ajuda/tutoriais-administrativos/ it says (roughly) "In DNS (OPTIONAL), you can inform the DNS servers previously configured for your domain. If you do not have this information, ignore this field. Our system will automatically use the DNS servers made available free of charge by Registro.br."