No, that makes sense to me. I'd say to also check out Cradlepoint routers. I remember the IT at my old company used them in all remote offices because they supported the usual connections plus had a couple SIM slots for LTE backup. They are probably not consumer friendly (designed more for IT pros) but worth checking out.

I looked a several options for a cellular router (Pepwave, Mofi, etc.) Ruled out standard mifi units (the little ones supplied by TMobile, VZ etc) because they failed to have good signal at my house.

I continued research and ended up with this Cradlepoint unit, model CBA850.
https://ltefix.com/shop/routers/crad...t-refurbished/

After I configure it I will report back.

Anyone tried it?

I've no experience with that particular Cradlepoint, but back in the 2010-2012 timeframe I'd owned several brands of their WAN-enabled routers and found them to work well with variety of modems, from HSUPD (3G) to WiMax to LTE.

I’ll just say it. There is no way that a cellular data connection is a better option than Comcast for this use case. Even if you’re not a heavy internet user, non-trivial amounts of data silently get consumed in the background as your devices update. Streaming video is really not an option when you’re paying per gig. Wired connections are faster and cheaper and more reliable than cellular.

Your Comcast bill for Internet only should be ~$50 per month or less for basic service. Ditch the expensive TV, renegotiate to a lower priced plan every year or two (threaten to leave if you don’t get a better rate), and definitely stop renting a modem and just buy one if you haven’t already. But going cellular is likely to be a poor choice here.

... honestly, WHS.

But if OP really only uses "a little" data, and has a grandfathered "unlimited" plan (which kind of does exist, I have an AT&T "unlimited" plan on a SIM that never seems to get throttled (but I have never used more than ~40GB), then I could see the use-case.

More detail on my current service. I have Comcast
with no phone or TV add-ons.
Cost here is $103.00 per month.

No doubt cable is faster and more reliable but not sure I need either. Just not at all sure it's smart, in my situation, to be paying $1,236.00 per year.
Plus I can always switch back.

Again, I have the ATT Mobley for $20/mo
and Google Fi for approx $60, both almost unlimited
data.

Nope, maybe, and nope. Doing a speedtest on T-Mobile right now on a Galaxy S7 (4 year old phone), 216Mbps down / 30.0Mbps up. Comcast is 28.9 Mbps down / 11.2 Mbps up. I'll talk about price below. And Comcast is NO way in Hades more reliable than cellular. I'm seeing 20-300 hrs/month of outages with them. It's been this way for years with them.

Depends where you're located and how much, if any, competition there is. Even in this area, the lowest Comcast internet-only package goes from $45-70. In many areas they have no competition, or perhaps a crusty 1.5Mbps DSL connection from the telco.

Google Fi is expanding its list of usable phones. Plus the ‘data only’ sim works with most anything.
I put one in my old iPad, works well, does voice calling too with Google Hangouts, WhatsApp, etc

It's not a matter of whether it'll work at all. It's more that unless it's one of those approved devices, not all functionality will work (namely carrier switching). Of course, depending on the person and use case, that might not matter.

Comcast connections go to a gigabit and beyond, so just because you didn’t subscribe to a particular service level doesn’t mean wired isn’t faster. Comcast’s second-cheapest tier is right up in the same neighborhood as T-Mobile. And you won’t consistently get 216 Mbps on T-Mobile either. Typical download speeds are well south of there, especially in areas that don’t have 5G coverage. I just did a T-Mobile speed test and got 35 Mbps down and 9 Mbps up on LTE which is far more typical. Tom’s substantially concurs on this, finding an average of 36/16 for T-Mobile, and Verizon the fastest at 53/18.

Even where I live the only other option is DSL. No Fios, no U-Verse, or other fiber offerings. My bill for TV and 200mbps Internet combined is the same as OP is paying for only internet.

20-300 hours/month of outages is far from a typical experience. 300 hours/month is 15 full days. Even if you typoed that and meant 30, that’s not common either.

I use LTE router by TP-Link TL MR6400.
Plug the SIM card in, hook up the unit via ethernet cable, install wizard did everything.
200+ days of uptime and counting.

I just ordered the Cradlepoint CBA850 (from LTEfix.com)

Hope its set up and config are not over my head.

Also hope I can get good signal and speeds
from my ATT Mobley and Google Fi SIM cards.

Not a typo, and yes, that *is* typical here. Due to how ugly things got with Comcast and one of my former employers, I have the phone number to the headend and a few of the chief techs' mobile phones. This is typical Comcast service for this area, even for business accounts, as their techs and a few local businesses can attest. This is an improvement over Frontier which left me with a total of >60 DAYS of downtime in a single year. I've had far better internet service in rainforests in 3rd world countries.

"Strokes for folks", and "location, location, location". I may hate their pricing, but with the exception of a gnarly pole tap caused by an errant squirrel last year, the only downtime I've had in 5+ years of Comcast are when they update my modem's firmware in the middle of the night- and my Gigabit connection is quite consistent to the point of never worrying about it.