Dual Sims - Dual Carriers - One Apple ID - How to merge data (including messages) and forward communication to new #.

I currently have an iPhone 6 (T-Mobile sim)and an iPhone 8 (AT&T sim). My GOAL is to merge the data into the T-Mobile sim and get rid of the AT&T line in favor of a less expensive T-Mobile plan. However, I've been using my AT&T (iPhone8) for years.

Yesterday, I swapped the sim cards (currently, the AT&T sim is in the older iPhone 6). Of course, my (AT&T current, most active phone line) texts/imessages are going to the older, unsupported iPhone 6 (Running the 12.5.7 OS)

I would like to have all of my text messages merge and reside on my T-mobile phone/sim/single apple id so I can access them from any iPhone/iPad/MacBook and direct all contacts to reach out to me on the new #. Is it possible to maintain the integrity/continuity of my texts? Due to the unsupported OS/feature limitations of this version, syncing the messages from the iPhone6 with the cloud is not an available option, as it is on the iPhone 8.

Feedback is much appreciated! Thank you!