Which goal gets served by the separate router? I've been thinking here for a while, but the only thing that comes to mind is a very restrictive "allow-list only" firewall.
Which dumbphone are you using? The majority thereof seem to be KaiOS based, which frankly is not sufficiently dumb for me to warrant the switch.
The router was designed to serve three purposes and I use all three of them: (a) to simultaneously connect up to five devices (smartphones/PCs etc.) to the internet via normal WiFi connection which it then routes to the internet via a mobile SIM card; (b) it's also a WiFi LAN switch in that it will allow local interconnection between the five connected devices; and (c), it has provision for an onboard SD card to which the five devices have access (i.e.: it acts as one's local mobile mini NAS). You'll see reference to detailed specs of the Hauwei R216 that I use in my previous post in reply to 1vuio0pswjnm7.
In my case, I use a fully-fledged reasonably current Android smartphone operated without SIM card and set to airplane mode for normal app usage, location and maps when needed, as well as internet browsing and non-Google email (POP/IMAP)—thus, the phone's only internet access is by either WiFi (to the router—my usual way) or Bluetooth—to another phone's internet connection (normally off).
Note: the phone is never used for telephone calls and it cannot be used as such as the router's SIM is a data-only type (that's to say one has a mobile phone number that cannot be used to make normal phone calls). Moreover my ISP, as many do, differentiates a data-only SIM/service from a normal one that does both. (In data-only services, one trades normal voice phone for extra data/cheaper data rates—you know, the usual ISP con job of artificially inflating a mobile phone's data charges. Nuking phone/voice access in data SIMs somehow—as if by magic—justifies ISPs to sell you data at a much cheaper rate. Furthermore, normal SIMs often won't work in routers for similar nefarious reasons).
As mentioned, I deliberately avoid Google services but using a phone this manner doesn't preclude one from doing so. I've found that if you use Google services, etc. then there's an added privacy advantage of disconnecting the phone from the actual telephone number as that now belongs to the router, moreover any app that that reads the phone's IMSI number will not be able to find a corresponding telephone number. I've several phones that I connect to the internet in this manner and every one of them has never had a SIM in it so Google is unable to link the phone's current ISMI-only configuration to any former IMSI/telephone number combination as there's never been one. Furthermore, in one instance when rooting one of my phones I accidentally formatted the partition containing the IMSI information, etc. and whilst I had the means of putting the info back I decided not to—thus apps no longer have even an IMSI number as an ID reference. Incidentally, this is still legal as far as regulations are concerned as the router and router SIM now provide the IMSI/phone number combination.
My phones also gain extra privacy from the fact that they're rooted, one can use the many Xposed Framework tools and such to improve privacy, nuke ads etc.
On the matter of firewalls, I normally use one on the smartphone itself rather than say installed in the router for purely practical reasons in that it's easy. The drawback of course is that if the firewall stops for any reason, which on occasions does happen (especially so after a full restart), then any apps that have a collection of data will use the opportunity to send it (my default is that no apps have internet access unless it's specifically needed as part of the app's function and the firewall is set accordingly—this also acts as extra method of nuking ads although I mostly use F-Droid's ad-free apps). This risk can be essentially eliminated with a rooted phone but I've not time to go into that here. BTW, I use several Android firewalls apps (not on the same phone of course) but I've found the easiest to use is Karma Firewall.
Re my dumb phone, I've been using an Aspera F28: https://asperamobile.com/phones/easy-phones/aspera-f28/ and its later incarnation the R30 but I'd not recommend them and they're unlikely to be available in many places. Their batteries are too small and of inferior quality and have to be replaced often (at least they're removable). Nor would I recommend other Aspera phones for similar reasons. I doubt that they use KaiOS, if they do then I've seen no sign of it. I reckon you're right to be worried about KaiOS especially so since Google has invested millions into the project.
Incidentally, I've other better flip phones such as Motorola ones that I can no longer use as they're only 2G (which is ideal for dumb phones) but unfortunately where I live they've now killed 2G. Doro dumb/feature phones may be worth considering as they've have always had a reasonable reputation (in the past I've thought about getting one but I've no practical experience of them). I know that Doro used to use their own OS but I cannot tell you much more than that except to say they do use KaiOS on at least some of their phones, the 7050/7060 for instance.
Of course, much depends on what you actually want to do. As I've mentioned in my previous post to 1vuio0pswjnm7 that carrying three devices instead of one can be rather inconvenient as there's more bulk to carry around and also the chances of losing one of the devices is potentially higher—one needs sufficiently large pockets to carry them thus size and bulk matters. As a person who's always carrying around lots of technical junk this is a hobbyhorse of mine and I'll address it in more detail when I reply to Iolaum.