There are fluoride gels designed to apply after rinsing that you might consider. E.g., CTX4 [1]. There is an over-the-counter version and a prescription version, with the prescription version having a much higher fluoride level.

Many dentists sell the prescription version, often without the formality of a prescription as long as you are a patient of theirs. They are maybe 2 or 3 times as expensive as regular toothpaste, but in my experience a tube lasts way longer than a tube of regular toothpaste.

That's because when you are just using them to apply fluoride at the end rather than to clean you don't need very much. Just put a small amount on a toothbrush and rub it over all the teeth.

[1] https://www.drugs.com/pro/ctx4-gel-5000.html

Not rinsing also leaves all the food residue, germs (dead or alive) and debris that has collected in your mouth throughout the day. Not rinsing my mouth after brushing feels like showering and leaving the soap on.