I concur that if the goal is to have a fully functioning phone that remains at home (similar to a land line) the phone should have its own number.
FaceTime and Messages can be configured differently on each device even if the devices are signed into the same Apple ID. You just need to set up the apps so they use the phone’s number instead of the email address(es) associated with the AppleID.
I do this with my personal and work phones; they both use the same AppleID but the work iPhone only receives calls/messages sent to and initiates new conversations from that phone’s number.
Taking a different tack to the desire for a home phone line. Depending on the costs associated with adding a new line to the OP’s cellular plan, an option that may be cheaper would be to set up a basic VOIP service. There would be up front hardware costs but the monthly service might be less than that of the additional cellular line.