I seem to have solved it.
Took look at the old.log after posting it. Googled error codes. One of them seemed to be WLAN related. The Asus motherboard does not come with WIFI, but I had an old unused M2 WLAN card. I had it in when installed libreelec, but I don't have antennas for it. It could not (unsuprisingly) find network without the antennas, so I took it out when I was adding SDDs into the case.
Decided to try putting it back in after I saw the error code, and it seems to have fixed the problem. So not finding that WLAN card caused LAN to also not work.
Sorry for bothering with such silly error. Appears to work fine now. Did three sleep->wake up cycles and worked fine after each one.
Thank you for the help!