Will he get a funtion for the system like temporar summon in exchange for something and he can chose 1 out of 5 options for a temporar help.
If he is strong enough he can go to a world like hunter x hunter to learn nen or mahouka koukou no rettousei, in that world they use tech to assisst in magic which can be useful to get more in magic and tech, also in hunter x hunter be honest with the president nero might be faster be telling him you are not from this world and exchange few things, he might be even be will to travel to another world just for the fun of fighting someone strpnger than him, and you get a good nen teacher and friend
If you need ideas just create a grupe or something so to ask for some ideas, some people do that and also for opinions
"Hughes" from, fullmetal alchemist brotherhood, summon him
Dont give him something to strong to early, or it wont be fun, give the maid something good to help him, and summon "spike" anf "L" from cowboy bebop, and death note...
Next time he need to use the "domain expande" he can try to throw the enemy as hight as posible in the air to avoid hurt anyone else, unless he get another domain...
abe no seimei, template from the anime "onmyoji" (netflix) ... onmyoji are mages, Exorcist, and fortune teller... and when it camo to fight demons that will be useful and they know magic.
He can use information of the novel lord of the mysteries, like the booms and others think to show he know things they dont...
kurumi - anime "date a live" she has time abilities and a ways to protec yourself is important, again time traveler and the time stone, specially the time administration, so having her as a template is a must.
Dont worry i like your story, try to add a recycle funtion, to his system for thing he doesnt want but the output is less than whar he put
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