I figure if my only reason to get an alternate smartphone is for this purposes, I might as well get a hotspot device instead?
My question is thus:
Why do you need a separate device and plan at all?It's great that you're only spending $15/month or so with Truphone, and I understand not wanting to use their PAYGO data. My question, however, is why not laterally move to another provider where you only have the one device, only one account, and save some money? Are you doing a lot of international travel? The price difference between what you're spending on Truphone now plus the costs and data plan on a separate device is going to at best match the cost of simply going, say Consumer Cellular which does permit domestic roaming on top of their network coverage (not data, though). One less device, one less bill, and you can tether the phone via USB as a modem versus using it as a WiFi hotspot, which negates the battery issue. (I'd still recommend checking on that with support first, but if they're still permitting WiFi hotspotting with iPhones, they should permit tethering as well - the data usage would be identical in your situation for your laptop.) Using a dedicated hotspot device isn't going to make the connection faster... real world, you're probably not going to see much faster than about 5Mbps down anyway no matter what you use.
If it's about international service options with a domestic MVNO, there's always the KnowRoaming sticker.
I can certainly understand too not wanting to churn and stick with a provider that's treated you well, but if the provider no longer has the capacity to fit your usage needs, then don't be afraid to make the change.
If you have reasons to keep it separate, okay, keep it separate. All I'm suggesting is, why buy more equipment, carry around extra devices, pay more, and deal with yet another monthly bill just to get mobile data access for your laptop?
You're already questioning if 2GB is going to be enough per month.
You've already stated that $30 is reasonable for internet only access.
You're already spending about $15 a month on average for only ~160 outbound billable minutes/texts/MB of data (not counting inbound minutes/texts).
AT&T coverage is going to be far more robust than T-Mobile.
You could spend $45/month split between Truphone and TOAST.net, get 3GB of data and the exact same ~165 units of billable service you're getting with Truphone, have AT&T coverage, have to buy and charge a separate device (since you seem to want a WiFi hotspot, that's another $50-ish), and add complexity and hardware to your travel routine...
Or you could spend $45/month plus a couple extra bucks in telecom taxes to potentially go Consumer Cellular, get 250 minutes of talk time, "unlimited" texting, 3GB of mobile data all on the AT&T network, and only have to buy a $10 SIM card down at Target, port your number, and keep using the phone you already have. (Reminder: GSM networks can handle data connectivity and phone calls at the same time.)
Make sense?
I know I advocate not churning needlessly, but a move of this nature isn't needless churning. We're talking about differences in electronic waste generation and demand, simplified billing and usage, and a volume of needed mobile data that your current provider simply can't offer at a competitive rate due to the structure of their business model.
Again, if you've got
reasons for not wanting to consider this route, fantastic... but I hope you can see why I'm suggesting this path from a purely KISS engineering perspective. I point this out, because it seems like (and please correct me if I'm wrong) you are already way overthinking it and trying to make it more complicated than it needs to be.
KISS your needs.