The _dmarc entry for my domain is already in use for another service (specifically: Brevo.de, a newsletter tool). In principle, only one _dmarc entry is possible in the DNS settings. However, I would like to send confirmation emails via WIX forms without having to route them through Brevo.
You don’t create a separate DMARC record for Wix. DMARC is domain-wide, so you keep the single existing _dmarc record (the one you use for Brevo) and just make sure Wix is properly authenticated under it. What matters is SPF and DKIM. Add Wix’s SPF include to your existing SPF record https://tropical-casino.com/ and enable DKIM for Wix (they provide the CNAMEs). As long as Wix signs mail with DKIM aligned to your domain, your current DMARC policy will pass and Wix form emails can be sent directly, without routing through Brevo.