Best way to integrate cloud sensors?

I’m trying to create a few sensors to monitor cars, garage, etc. - none of which are within wifi range of the house. Intended hardware is Arduinos/ESPs with GSM modules. My setup is HAOS with Mosquitto and Node-RED, currently without remote access.

The obvious way of receiving data from these sensors would be to pay for Nabu Casa, which would expose the necessary HA webhooks without poking a hole in my firewall. However, if I didn’t want to pay an ongoing cost just to receive a few kB of data per day, I was wondering what the current best practice would be?

I’ve been looking into the various cloud IoT platforms as a means of achieving this, but none of them seem to offer simple integration with Home Assistant while having a free tier. The closest I can find is the Google Pub/Sub integration, as well as various Node-RED modules that seem to be updated sporadically or have even disappeared entirely.

Maybe I should use Node-RED to connect to a cloud MQTT broker and relay the messages to Mosquitto?
Or, if I should just open up remote access to HA, what’s the best method given this use case?

Many thanks.