I’m going to answer this two ways, the first is from the reverse of what you said - I went from a MVNO to a Primary carrier, and then as a former employee (Tech support) of a major carrier.
So first, going from a MVNO to a major carrier was a huge step forward (improvement) in customer service, specifically. In general, if something was wrong prior to that, it was reboot your phone, check the signal, sorry, must be the network. Done. I won’t say you get what you pay for, but if something was wrong on anything but the billing side, they had NO idea, and since they couldn’t/wouldn’t contact the bandwidth owner for any info, you were SOL for reasons and ETAs.
The second issue I had was already touched in the older portion of the thread. If I was at a Con or other major event with lots of people in a little area, everyone got crappy service, but I got nothing but ‘unable to send’ or ‘unable to connect’ the majority of the time. So yeah, there are problems on both the quality of service and customer service.
Okay, now answering as a former employee who knows the options. You don’t have to go all or nothing. The major carriers do have less expensive ‘pay as you go’ plans, that are less expensive and less feature rich than the primary plans. You WILL get less service, because the prepaid/pay-as-you-go plans are less attractive and get less support, but they are a lot cheaper with fewer of the issues of the MVNOs.
So, since I worked for T-Mobile, there info is easiest for me to find. This would be a good option for at least the 3 who don’t use a lot of data -
https://prepaid.t-mobile.com/plan-detail/t-mobile-connect
In short, $15 per month (before taxes and fees, and you BYO phone) for unlimited domestic calls and texts with 2.5G of high speed data. And then get a single line for your son with an extra 3G of data (5.5G total per month) for $25 per month.
Total bill before taxes and fees, 3x$15 (45) + 1x$25 (25) = $70 per month. And it’s still a major carrier, but it’s a hard cap on the data, and you will be in the ‘no frills’ category for flexibility. You’re not going to see phone trade-in offers, upgrades, perks or the like with this. But from your final line, I doubt you’d see a change in phone usage/quality of experience while still likely saving quite a lot of $$$.
Hope it helps.