Thank you to Michael (dg0opk) who wrote in and wanted to share details of his full SDR monitoring system for weak signal HF modes. His setup consists of nine ARM mini PCs (such as Banana Pi's, Raspberry Pi's, and Odroid's), several SDRs including multiple RTL-SDR's, an Airspy Mini, FunCube Dongle and SDR-IQ, as well as some filters and a wideband amp. For software he uses Linrad or GQRX as the receiver, and WSJTx or JTDX as the decoding software, all running on Linux.
Michael also notes that his Bananapi FT8, JT65 and JT9 SDR monitor has been up and stably running continuously for half a year now. Bananapi's are lower cost alternatives to the well known Raspberry Pi single board computers, so it's good to note that a permanent weak signal monitoring system can be set up on a very low budget. Presumably even cheaper Orange Pi's would also work well.
With his setup he is able to continuously monitor FT8, JT65 and JT9 on multiple bands simultaneously without needing to tie up more expensive ham radios. His results can be seen on PSKReporter. A video of his RTL-SDR Raspberry Pi 3 decoding FT8, JT65 and JT9 can be found here.