We (2x household) have a "third" phone in addition to our work phones. We use it for personal use that we don't want on work phone and on vacation. So, use is low to moderate. currenlty, we still have the iphone 6 and need to upgrade. At the same time I like to review the plan. Also though that cyber Monday is a good time to look into this.
The phone line is rather old and we have a grandfathered unlimited data plan (however, can't use for hotspot). Otherwise we have a Nation 450 with Rollover plan.
The plan comes to $70.44 + taxes/fees.
I don't see you finding a better deal for unlimited data. What is really funny is that if you check some of the "unlimited" plans carriers offer, there are several options for it. I don't think they know what "unlimited" actually means LOL Verizon has 4 unlimited plans, that's just silly, it's like having 4 tallest mountains in the world.
Then it's really up to you and if you want to keep the same phone, what phone you want to get and what service works best in your area. In my experience Verizon has the best coverage and signal of anything else I have seen. There are a dozen plans with any service, no way to pick one for you since it's up to you what you want to pay and for what type of data. For a phone, no way would I pick anything over a few $100 for a "backup phone". If you like Apple, find a used 7 or 8, or get an Android, plenty decent models for a few 100. Motorola G7 for example.