aria2c daemon start at boot. Put this file into /Library/LaunchDaemons, them 'chmod 600' and 'chown root'. Remember to fill placeholders below.

homebrew.mxcl.aria2.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>KeepAlive</key>
	<true/>
	<key>RunAtLoad</key>
	<true/>
	<key>Label</key>
	<string>homebrew.mxcl.aria2</string>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/local/bin/aria2c</string>
		<string>--enable-rpc=true</string>
		<string>--rpc-secret=<your secret></string>
		<string>--rpc-allow-origin-all=true</string>
		<string>--rpc-listen-all=true</string>
		<string>--rpc-listen-port=6800</string>
	</array>
	<key>WorkingDirectory</key>
	<string>/Users/<your username>/Downloads</string>
</dict>
</plist>

this is great, i was looking for this

Thank you very much this one works like charm!
BTW, if you are using Homebrew Aria2 instead of put this file into /Library/LaunchDaemons, you can put this file into /usr/local/Cellar/aria2/VERSION/ then you will be able to control it via brew services cmd.