Cross posting in multiple channels
It easy to use, and accessed through the "rich text" button on the bottom left when you create a new message. Then select your channels in the "select channels" button.
Be aware though, once these are posted, there is for now no way to see which channels you have posted the message in, and there is no aggregated view of all the comments in the different channels, they have to be monitored individually for now.
This is the M365 roadmap ID 52393 and should reach all tenants by the end of this month.