John
I've been carrying a cell phone for 23 years, and over that time, I've had a number of different phones - including some 'flip phones'. So I can say that I've had some experience. One thing that I will say is that I don't care for 'flip phones' because there is an additional (and unnecessary) action required at the beginning and end of each call.
There is a distinction between a 'cell phone' and a 'smart phone' - as Rick says, a cell phone is just a phone that you use to talk to people on.
A smart phone is actually small computer and introduces many new functions. I can understand that the initial reaction is to say 'I don't need any of that stuff', but my experience is that once you have it, you will be amazed at how much you use those additional functions. The most obvious is the camera, but the ability to access the internet, send and receive e-mail, and do navigation are other extremely useful features.
That said, smart phones come in two major but not clearly defined groupings - the expensive phones and the cheap phones. If you are a newbie to smart phones, it might make sense to start with a cheap phone knowing that you may need to upgrade at some point. Two years ago, when we had to upgrade my wife's phone (similar situation to you - it was a 3G phone that was about to become obsolete), we opted for the least expensive LG phone we could find knowing that historically, she rarely used her cell phone. So rarely that she had to ask me how to turn it on each time she used it! But after making the switch, she found that she uses the new 'smart' features even more than she uses it as a mere phone. So my expectation is that when the time comes to replace that phone, we will be looking for something a little better.
Three things to keep in mind. First, the life expectancy of a phone is essentially governed by the life of the battery, and when you reach the point where the battery can no longer be recharged, you must replace either the battery or the entire phone. And at that point, it may be easier to replace the phone than to replace the battery. Ultimately, battery life is determined by how many times the battery is recharged. Apple claims that they design their phones for at least 500 recharge cycles, and the typical Apple phone users reaches that threshold in 3-4 years; I have not seen information from other manufacturers, but I would assume that they aren't all that different. One data point: I had to get a new phone in January of last year, and I reached 233 recharge cycles after 505 days, so I expect to reach that magic 500 cycle point around the end of 2023.
Second, don't be fooled by the marketing hype around 5G. 5G is mainly vaporware at this point, and even when it is fully implemented in the distant future, it won't make any difference in basic phone functionality, and very little difference on most smart phone functionality. About the only practical impact 5G will have will be on the ability to do things like watch Netflix movies on your phone. For the foreseeable future (perhaps 4-7 years), 4G LTE will be just fine.
Third, because smart phones are small computers, there is a choice between two operating systems - iOS and Android. iOS means Apple phones - they are good phones, and the smart phone functions seem to be integrated more logically and efficiently than Android, but you do have to pay a little more for that convenience. Android phones work just as well although some of the smart phone functions don't work together as seamlessly, but that's something that you can learn to live with. There are more Android phones to choose from, and with a broader range of prices. You pays your money and takes your choice. Samsung is a great brand, but they are a bit more expensive. Google Pixel phones have a great reputation and can be had for a bit less, and a brand that has gotten a lot of attention recently for good quality at a reasonable price is OnePlus.
And one final point - the phone and the service provider are separate things. You can use most any cell phone with almost any cellular service provider. There are many cell phone service providers in the US, but only three actual physical cell phone networks (AT&T, Verizon, and T-Mobile). Service providers such as Ting, Google Fi, Spectrum and Consumer Cellular lease space on one or more of the physical networks. Ultimately, what this means is that any phone should work with any cellular service provider, at least technically. However, specific cellular service providers may offer only a limited number of phones for sale/lease, and if you choose to purchase/lease a phone from a cellular service provided, you may be locked into using their service for some contractual period. So shopping at independent retailers such as BestBuy, Target or even Amazon may offer the broadest selection of options.
