lm-sensors... you mean the CPU temperature could be a problem?
The collected data in collectd doesn't show any issues with the CPU temperature:
https://i.imgur.com/KO9gzNF.png
Also weird: Tvheadend reports only SNR of -13 dB. A "normal" satellite receiver reports both a signal strength and signal quality (which is the SNR?) of about 70%.
Previously I used the ddbridge driver included in the Debian kernel (I think it was version 0.5) which also reported the signal strength and SNR in percent. With the old driver I got an SNR of about 25% and signal strength of about 70%. Therefore I don't think it's an issue with the coaxial cable, F-connectors, LNB or the dish. Otherwise, the satellite receiver also would have an SNR of about 25%.