Hello Kai,
I can't be of much help here, but I don't think any floating plant would be able to survive in brackish conditions. Since you already have Hornwort thriving, it will be a good floater in place of the "traditional" ones like frogbits, etc.
I found this online guide to brackish aquariums with various setups and easy to follow guidelines per setup version.
http://badmanstropicalfish.com/brackish/brackish.html
You will find acceptable plant species capable of surviving each type of brackish setup in each section.
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