I don't know of a platform that specifically caters for this. Would be interested to see what others suggest. I've found opportunities locally at game stores, Discord, through meetup, and also through Reddit (apologies Lemmy but there is not enough user base for a city based ttrpg comm here).
Meetups is good if you run a regular session and are open to drop ins Discord and Reddit or some forum styled website would be better to fish for interest in something, but people's schedules and priorities change so you will have to ask regularly. Not much you can do about this. I don't see this as the platform's fault, rather the nature of what you are trying to coordinate.
If discord messages fading too quickly is an issue you can create channels for the different games you are trying to run, or give each GM their own channel for whatever they're pitching, and pin a message in each so they don't get buried into oblivion. I have joined servers that use this system, not for ttrpg admittedly, but this functionality solves that issue.