The first stop in my search 
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Don't see any devices with local integrations, i.e. ZWave/Zigbee/Matter, but there is an older Airthings View Plus driver that should still work.
Might be able to use a detector with Bluetooth if you have a C8-Pro. Don't see any integrations already built but with a little assist from AI it might be doable.
I have yet to dip my toe in the AI pond. However, I do have HA up and running. So, that might be an option. I'll be poking around looking for something.
I thought about integrating radon readings into home automation, but then asked myself why I’d do that. Radon levels vary dramatically day to day, and any one data point is virtually meaningless. A rainy day has a profound impact on radon levels, as does seasonal changes and other factors.
What matters is radon level over time … years, really. But anything less than an average over a week or month just isn’t actionable. Unless …
If you have a radon mitigation system, it might make sense to crank up the fan on bad days and down on good days. I tried to find a variable speed controller for my radon fan, but could not. The closest thing would be a 25%/50%/75% fan switch but for some reason that didn’t work out, but I don’t remember why.
I do have a fan. I was thinking of cycling it in some way to save energy. The basement is unoccupied. I have a radon sensor down there now, but it is not smart. The levels definitely go up when the fan is off. It draws 17 W so it's not huge.
Don’t. Your health is worth it.
I remembered why. I tried a 25%/50%/75% fan switch but it didn’t give me much granularity in fan speed. 25% was almost no fan at all, and 50% and above were pretty much full blast. I would need a continuously variable smart control.
Side story: I have a house with a crawl space, and the crawl space is encapsulated. The air in the crawl space goes through a dehumidifier to keep moisture levels down. But here’s the thing: the radon mitigation sucks out air from the crawl space. On a hot August day, the dehumidifier runs constantly pulling in make-up air, which the radon fan then blows out. It’s expensive to air condition the neighborhood.
Radiation damage is all about cumulative dosage. There’s a huge difference between sporadic exposure (say once a month to Spa) v/s chronic exposure.
Sporadic exposure gives your body’s DNA repair enzymes time to fix DNA damaged by radiation. Chronic exposure severely reduces that time. Therefore, mutations are more likely to be “fixed”. Mutations that activate growth pathways are pro-oncogenic, as are mutations that inactivate tumor suppressor pathways.
So my advice stands - your health isn’t worth the small savings.
Nevertheless, it would be nice to be able to get radon readings into Hubitat.
I use this, and it works well - It’s cloud based, but I only get updates every 20 mins or so.. - The AirthingsView Plus monitors alot, I purchased it for Radon measurements in my basement, but they are consistently low, and now I focus on using VOC measurements in a art studio (in said basement) to turn on an exhaust fan.
It does the usual Temp, CO2, Humid - But the PM measurements are mildly interesting, and it even registered higher than normal particulates when we had a local forest fire, it a national forest around 10 miles away.
So the Airthings view is not the cheapest device, but it measures alot of valuable Air Quality data, over the long term, and the “near real time” Radon and VOC measurements are the major draws for me. - I just wish it natively supported Matter or MQTT, but it is what it is. Not sure if the cloud connection is a show stopper, but it’s just a monitoring tool, that has been well behaved for me over 2+ years now.
Having multi-year graphs of Radon for a basement, is definitely a plus when trying to sell, and get past home inspection requirements. - Images attached for 30 days of Radon measures (it stores a year of data on their cloud), and Overview of all measures - They also send me a weekly “summary” of anything that looks abnormal. My wife works in the studio, 6-8 hrs a day, so the extra data and visibility is worth it to me.
Cloud is a turnoff, for me.
To each his own, and I’m not a fan of cloud for actual control variables that drive automation, but I’m OK with it for monitoring, as well as “accessory devices” (aka Roomba voice control). But I get your point, and agree for more critical HVAC and lighting controls. If I get 5 years out of the device it’s worth it for me, given peace of mind, given that Radon is common (and varies house by house) in my area.
I’m gonna give the Wave Radon a whirl.
Should be local with HA.
I’ve also been very happy with AirThings’ Radon measurements. I have 2 of the View plus devices, and also bring them in via the awesome cloud interface. I wonder if someone will bring the HA driver in HE eventually… I suspect it would be doable with the help of Claude or another AI, or by someone who is good at these things… 
Think the issue is that the C8-Pro only supports BTHome v2, while the Airthings devices are using BLE.
Is that really only Shelly devices at present?
Shelly and some ESP32 (might be an option there for you...)