mcrcon VS mc-rcon-client

This could possibly be done with Skript using yaml to tell it what to do on startup or shutdown. Although I would personally prefer using RCON like u/Mutated_Zombie mentioned. There exists cmd execs you could use in a batch script.

If you can't find a tool to do it the way you're looking for, my approach would be to install mcrcon on the server and then SSH to the server and run the commands to grant OP via the terminal. (Plenty of SSH clients for the phone, if you need to do it remotely.)

Note that if you run MC this way, you need to connect via RCON, via something like mcrcon.

Why would I use this over mcrcon?

You can use rcon to run commands and filter the output. It's not pretty, but it doesn't require any kind of external service (also you need to write the scripts yourself).

https://github.com/Tiiffi/mcrcon this is the rcon tool.

There's no easy answer to this. To do what you're looking to do, you're gonna need to write some code, either a separate plugin that hooks into their API (if they have one), a custom version of that plugin built from source with your own changes (if their code is available publicly), or otherwise some clever business with some sort of external tools (maybe mcrcon?) and some scripting language like bash or python.

If running on Linux, you could use a use mcron and add that to the systemd script

yep this works. I personally use https://github.com/Tiiffi/mcrcon in my scripts

use mcrcon