Did you choose a KaiOS phone specifically because other current non-smart phones lack support for Bluetooth tethering (a.k.a. PAN profile)?

As far as I can tell, all currently available models from all manufacturers are based on some Unisoc platform and offer no indication of support for this feature in their manuals. Did you happen to come across any alternatives?

I'm not very keen on KaiOS given the ubiquitous advertising baked into it (which is apparently their business model).

I don't really use Bluetooth, it can pair with my car to do hands free calling but have never used that.

Got it because it was a small, low-cost, dual-sim phone that could also act as a router. It replaced an even lower cost Nokia feature phone.

Don't run apps often enough to be bothered by any advertising.