How to sync phone to Fitness app after unpairing Apple Watch?

I used to have an Apple Watch that would populate data into my Health app and Fitness app. I've since unpaired the watch and only use my phone for step tracking, etc. However, my phone steps are not syncing to the Fitness app even though they show up in Health.

I've followed previous instructions to ensure that my phone is the first option in Data Sources on the Health app as well as restarting the apps/phone. I've seen past advice to delete all unused devices from the Data Access and Devices section in Health settings. However, I am concerned that if I delete my Apple Watch, then I will lose all my old Fitness data.

Does anyone know how to make your phone start syncing to the Fitness app after unpairing an Apple Watch? (IPhone 16 Pro Max)