12v key on problem

Time to start the process of elimination. With key off, Use a remote starter trigger at the relay just to spin the starter while reading voltage at the battery. Then turn the key to run, use the remote trigger to crank while reading voltage at the battery. Check voltage at the relay and up the line. I’m betting you have a bad connection(s) somewhere leading to under the dash. Excessive Resistance somewhere. Inspecting the starter cables and wires coming from the battery to the relay and or bulkhead is a good place to start to verify no corrosion etc.