It's possible to swap SIMs on the T-Mobile network, and use the same phone with 2 different SIMs. So, in theory, that part is workable.
Phone SIM slots aren't really designed for constant changing of the SIM card, and you run the risk of wear on the contacts. In general, frequent switches are not recommended.
Depending on the phone, you might have to manually re-select the APN each time you switch SIMs. (Probably not a big deal.)
If you have the free plan, without paying the $5/annual charge to exempt your number from being reclaimed, you'll have to remember to use the number regularly to keep from losing the number. If you have a PC/laptop, you can do this in a browser window by sending a text via the browser, instead of having to swap the SIM cards just to make that regular use requirement. Exactly how long you can wait between use seems to vary according to whether you're in an area with high demand for numbers from TextNow.
I don't know if the TextNow app will remember the login info for the SIM card when you uninstall/reinstall the SIM: you might have to log back in to the app each time you swap.
Whether or not it's worthwhile to do this really just depends on how much you need the extra minutes without depleting your cellular plan. I'd be tempted to look for a cheap phone that you could leave the SIM installed in, since you'll get more use out of it if it's easy to use and ready to go. A cheap phone will work since you're only using it for calls/text. The usual waning that a phone with T-Mobile's newer band 71 will have better coverage in rural areas, either now or in the future: but I don't have any issues in my city with a phone on T-Mobile without band 71 (or band 66).
For what it's worth with respect to sending a text to keep the number...I currently still have TextNow service on an old FreedomPop LG tablet and haven't switched to the new SIM yet, but I have kept my number without either paying them or using that tablet to do so. I have the TextNow app on my daily driver, and whenever the TextNow "you could lose your number" reminder pops up I send a text back and forth between that and my Talkatone number. However, since I'm still currently on Sprint service, I can't vouch for what the app does if you try to switch back and forth between having the SIM installed and just using the app without it.
I actually had a TextNow number before they had service plans, if I remember correctly - I believe because I had gotten them mixed up with something called TextFree which seemed to be one of very few "second number" apps you could reliably dig SIP credentials out of.
I don't think I ever ended up installing TextFree and now I can't remember why not. Based on what I vaguely recall about the timing, it's possible that it was because that was around when I got my Obi, and registering it with Google Voice mostly eliminated my use cases for SIP credentials I could place calls from. It's also possible I determined the TextFree information was out of date by the time I stopped trying to extract TextNow SIP credentials. Neither one's relevant to this thread, though, I reckon, so I'll shut up now.
Oh, please don't shut up. I love learning about ways to make things work, and what works for different folks. If you're concerned about derailing a thread, start a new one. SIP credentials are currently somewhat over my head, but no reason they need to stay that way!