Hijack? That's the entire purpose!
Consolidation of information.
There's somewhat better PAYGO options than Carlos Slim around the same prices. H2O Wireless is one of them on the cheaper end.
As for phones specifically, I'm guessing you're talking Android given you already "know" what apps you need. Understand that even with the improvements on data management, it is still one of the most technically complex smartphone platforms to lock down random mobile data usage on to keep the ongoing apps you need mobile data for usable... this results in bills that may not be so friendly toward the pocket on PAYGO mobile data charges, resulting in you needing to leave mobile data off most of the time without potentially wasting random mobile data on needless services that Android is too stupid to prevent. It's not impossible, but it's a headache compared to how Microsoft has things set up on Windows Phone... though that is a path that's losing traction, too. (What is it with me and failed smartphone platforms? The least terrible ones never gain the traction they need to survive.)
Long-story-short, smartphones and PAYGO data are getting increasingly difficult to mix unless you just leave mobile data off most of the time.
As for what sort of Android smartphone you should buy, buy one with a user replaceable battery, an SD card slot, and that is mainstream enough to get system updates for more than their initial launch version of Android and/or be able to get further updates using third party Android builds such as LineageOS or Resurrection Remix (if you're inclined to do such technical things).
PhoneScoop has a good search tool for finding phones, as does
GSM Arena. Otherwise, you might be better served with a feature phone. I'd stick with GSM providers who allow for bring your own device, though, as your selection and pricing will be broader, and you won't potentially be locked into a carrier that you'll hate. Also, resist the urge to spend more than $150-200... and honestly, if you don't mind used/refurbished, you can get far cheaper. I use a Windows Phone myself (a Lumia 435) and I spent a whopping $18 for it a year ago. Solid, nigh indestructible, does everything I need, barely uses 30MB a month just using data sense without keeping mobile data off all the time and still gives me regular email updates.
If you're in an area with good T-Mobile coverage, consider using US Mobile for your carrier - good prices for the packages offered, data especially. Figure a budget of at least around $10 no matter who you use if you're wanting a smartphone with some ongoing mobile data usage.
Also, you brought up iPhones. These are money pits, and Apple dictates which carriers you can use unless you want certain core phone features broken, eliminating some of the cheapest and best coverage PAYGO MVNOs you could possibly use. If you actually care about saving money, don't buy an iPhone.