1. Getting the IoT traffic to your proxy machine. As you say this is fairly trivial when you can set a proxy, but a bit tricky otherwise as you need to run mitmproxy in transparent mode (https://docs.mitmproxy.org/stable/concepts-modes/). With some luck you can set a custom gateway on your device which simplifies the setup, otherwise you need to have the capability to set iptables rules on the router. I personally have a small $20 OpenWRT router dedicated for that. Alternatively you can run your own DNS server and point everything to mitmproxy. v7 now also supports SNI/host-header based proxying! The good news is that this setup is the same for all your devices and once it works, it works.
2. Getting the device to trust your (personal) mitmproxy CA cert. This one really depends on the particular IoT device and may involve some serious reverse-engineering if you are unlucky. I wish there would be a more of a "I should be able to inspect my own traffic" movement, but it's a tricky problem to solve.