What I Have and What I Want - but what's it gonna cost???

I don't want to frame this as a "Can this be done?" question, because I know that it can.
It's more of a "How much will it cost me per month if I do this? And how much up front? What promos are available to help get it done?"

What I Have:
Current everyday use phone is an S21 FE with my 25+ year phone number.
Home internet/wi-fi with Xfinity.
On the Mobile Plus plan with 1-year promotional credit applied (one year free). With add ons, ~ $16/mo now.

S21 is paid off and unlocked.
I do not want to trade it in (see below).

What I Want To Do:
Upgrade my everyday use device to an S26+.
Finance/make payments on the S26 - not buy it outright.
Port the established 25+ year number to the new S26+.

Assign a new phone number to the S21.
Use S21 as a cheap-y second line with the new phone number (unlimited talk/text; very minimal data). 

Ultimate Outcome Sought:
1.  Upgrade to S26+ with my established 25+ year phone number assigned to it.
2.  Use S21 FE as a second line with a newly assigned phone number, primarily for texting.  

Is anyone on here from Xfinity that could get me that info? The AI assistant was useless because the questions were too complex. 

Thanks!