A number of suggestions & ideas

[rayblon]

[05:05:33 INFO]: (FAWE) [FAWE tick-limiter] Detected and cancelled physics lag source at Location{world=CraftWorld{name=CityWorld_destroyed_nether},x=100.0,y=76.0,z=-201.0,pitch=0.0,yaw=0.0}

Been getting lots of things like these with FAWE installed on the latest 1.13 builds. FAWE seems to prevent any crashing when you have a high enough timeout time on the server. even if the lag still repeatedly boots you as it generates. I don't know what these are, but it seems to show up fairly regularly in the logs as I pregen with worldborder. Whatever these things are might be a source of the lag as of late.