Hotspot usage for "8 hours a day" will definitely burn through battery quickly. If you can only use it in short bursts, that's one thing, but if you want all day, dual connectivity, you'll probably do better to stick with both being cellular. The Tmobile option is only $10 for 5GB or 5 months. A sometimes overlooked bonus with the cellular option even if you don't use 5G service is that you also have GPS.
In the interests of saving money though, consider doing what I do: make cellular mini double as your phone. VOIP app plus buds turn Mini into a mobile phone with cellular service that will cost far less than iPhone service (I spend only about $25 per YEAR for 5G service). Telephony is not a form factor but only an app... just like a flashlight (app), camcorder (app), iPod (app), tape measure (app), map (app), etc. A Mac or PC can easily be a phone too in the very same way.
Doubts? Since you already have both devices, download a VOIP app to your iPad. They usually give you a free number. Then call yourself and call yourself back from each device. Text yourself and text yourself back from each device. You'll quickly realize that basic telephony works just fine on cellular Mini. If you decide to go for it, you can port your existing number to become your Mini phone number.
Bonus: the curious "long in tooth" effect that drives iPhone replacements every few years seems to take hold much slower on iPad Mini. I got 8 years out of Mini 2 this way and expect to get about the same out of Mini 6. For many, 16 years is about 5-6 iPhone purchases, all at prices higher-to-much-higher than iPad Mini. And again, then comes ongoing service costs, much higher than iPad Mini too.