String type - cut a part of

OK, given this is what you are truly after, I’d recommend linking a DateTime Item to one of the other MQTT Channels (or all of them) and apply the Timestamp profile to the link. Any message received on and of those Channels will cause the DateTime Item to update to now.

No need for any rules with this approach.

This is never ever required. If you can’t figure out how to avoid this you should come here and ask. You should never have to make more than one such rule.

See Design Pattern: Associated Items for how to handle any number of such Items using one Group and one rule.