I'm in my early 30's, and I also grew up with nothing but a landline. Cellphones are still more recent. I actually had a landline even as an adult, up until 7 years ago. The reason I switched to only cellphone is, after doing research, I found that the cheapest landline service actually cost MORE than the cheapest cellphone service. I don't know what landline service costs nowadays, but at the time the cheapest service I could get was $20/month ($240/year). I pay $100 for an entire year's worth of service on my phone. But I don't use the phone much, so that's why I can get away with such cheap service.
You don't need to pay for TV service to get cable internet, though you might want to look into packages that do include TV. I have some cable channels because the particular deal the cable company had cost more for internet by itself than internet + some TV. You need to compare pricing. Also keep in mind that the promotional deals' pricing tends to only last for about a year. Then the price goes up. Read the fine print. The particular package I have I would not have gotten if I was planning to live here long term. But since I only plan to live here for about a year, then it's cheaper for me to go for that package.
Also keep in mind that for cable internet, you really should look into buying your own cable modem to save money. The cable companies will charge you a monthly rental fee to use their cable modem. It will only take you maybe 7-8 months' worth of service to break even with the cost of buying your own cable modem vs the rental fees, and then every month after that you're better off.
You will probably be better off if you drop the landline and have cellphone + cable internet only. What you should sign up for really depends on your needs.
I can't say anything about Republic, Google Fi, or Ting. I have no experience with them. My phone service is a pre-paid, no frills service with Verizon. I pay for $100 worth of credit to be put on my account once per year and I never use all of the credit. You can't pay for less than $100 because the expiration times makes it not worth it. The $100 credit lasts for a whole year. The smaller amounts of credit, if I remember, only last 90 days. My service works very well for me, but I would not suggest it for anyone who uses the phone a lot.