Backing up an old iPhone

I've been using an iPhone 6s Plus, but it's gotten laggy/glitchy, and the battery is shot, so I bought a new iPhone 11. Unfortunately, I hadn't backed up my iPhone 6s Plus to iCloud in years (what was I thinking??). I'm working on getting the old phone backed up now, but it's been several hours, and it estimates that there are still 8 hours remaining. If I were to activate the new iPhone 11 without transferring any data/restoring anything, could I still work on backing up the contents of the old iPhone 6s Plus as long as it's connected to my wifi? It would no longer be hooked up to any cellular data, and I'm just not sure if that matters when it comes to doing backups.

Any help is appreciated!!