Next idea for this is what if the characters from Gravity Falls where in Cheldea. Now ground rules are mostly set in place so that Dipper takes the place of the main character in grand order but the other characters survive.
RULE ONEThe people of Gravity falls must either be unable, unsatisfactory, or just not that good at magic to be a master. RULE TWODipper and Mabel are exceptions to rule one this way the president still looks down on them but they can be present. RULE THREENo downplaying a characters faults, such as the fact Dipper overthinks things, Mabel thinks the world of herself, the fact both have problems with the opposite gender wither that be stuttering awkwardness or just essentially being a "slut".
The reason for rule three is so that Mabel in her silly uncaring state will just plain miss the transfer unlike Dipper meaning Cheldea keeps a "competent" master and she will not directly interfere with Dipper being on the mission. Due to the lack of staff in Cheldea the characters of Gravity Falls will take up the jobs such as cook, machanc, security, odd jobs like this that a servant is unneeded for.
About the servants, and this is where it gets a little strange. Since the game runs on a gacha system for one character some things will change. For one artifacts must be used for the summoning, these artifacts must be rare due to black key summons duplicate servants two people having the same servants. Summoned servants will be dependant on where/when a character has been placed on a mission since they will be summoned during a mission. For example if Dipper where to go to London he has a chance to summon Jack the Ripper, Mordred, Lancelot and so on. Servants will not be able to "ascend" like in the game, this is due to it just being a change of clothes.
Now about personalities, Dipper should be more accustomed to being away from "people" and more used to servants due to being a more reliable master than Mabel he would be sent out more often. This also means he gets more servants than Mabel due to gathering more artifacts by being out more. This also makes Mabel and more mediocre mages like what Robbie will be jealous of him either whining to just assaulting him. This would make him more interested in servents thus spending more time with them and raising the bond levels of each.
Family wise his parents and Grunkle's will be proud of him for doing so well even if some of his servants have quorks such as introducing themselves as his wife/teacher/big sister/brother and so on. Mabel however will be feeling betrayed and try to get him to either bring her or let her go in his place however his servants will give him the confidence and push him to say no to her unless the situation is serious.
The events for this story don't need to follow those of the game this is to give the writer more freedom in which servant joins.
As you all can see I have set it up so Dipper can have and will be comfortable with a servant harem. Don't care what anyone says harem is best genre live with it