See MQTT2 and Tasmota, contact transformations don't seem to work - #20 by rlkoshak for an approach that might ease some of the work for you. It’s kind of bare bones instructions so if something doesn’t make sense I can help.
With MQTT 1 you put an MQTT binding config on your Items. It looked something like { mqtt="blah blah blah" }. With 2.x version bindings you “link” Items to Thing’s Channels. In a .items file it looks something like { channel="mqtt:blah:blah:blah" }. But you don’t have to use .items files. You can create the Items and link them to Channels through PaperUI.
Do you have Simple Mode turned on? If so, I recommend turning it off immediately. You will have to eventually and the longer you postpone doing so the more work there will be to fix what Simple Mode has done. When Simple Mode is turned on, Items are automatically created for every Channel and they are automatically linked to those Channels.
So Channels get linked to Items. They don’t get linked to each other.