I think that you’re right about heat emitted from the phone. I found NXP power consumption measurements PDF for i.MX 8M Quad (temperature measurements start on page 13), and it seems to me that the SoC is running at least warm, when it performs basic tasks.
This is not really surprising, at least to me, as i.MX 8M is intended for automotive. I expected from the start that battery life will be noticeably shorter, compared to mainstream Android phones, and that it will run warm most of the time. I guess there is still room for some optimization on the software side, but nobody should expect miracles.
There is no point in complaining about these compromises. We can either accept them and make the best of it, or get a different phone.