So, how does it happen?
Choice one: Magic necklaces that bind girls to Ash are created by Azelf, Mesprit, and Uxie. Their magic powers are based on the abilities of the Lake Trio.
Azelf: Weakens the wills of the girls towards Ash.
Mesprit: Amplifies her romantic feelings towards Ash, and suppresses them to other boys.
Uxie: Knowledge of where Ash is at all times.
The necklaces also use the Lake Guardians' shifting between planes of existance to teleport to other girls one hour after becoming permanent. As a girl's mind is altered, she goes into an autopilot mode, where she seeks out Ash, all the while, the only thoughts actually going through her head being romantic fantasies of them (as well as positive memories of him, if she has them). She'll only leave autopilot after she finds Ash. She'll resist having the necklace removed until it is permanent.
A girl under the effects of a necklace has her pupils disappear, only Ash and the girls effected can notice. The girls will always see it, but Ash will take three days after the creation of the necklaces to be able to see it, however, he will be able to tell something is wrong before that, just not put his finger on it. The necklaces are made of Red Chain, with one having a blue gem, one having a pink, and one a yellow. Chloe, Dawn, and Serena are the first girls to get them, and the necklaces put out a low powered mental wave to give the girls holding them a compulsion to wear them. If Ash has a necklace, it won't teleport unless he wants it to.
For choice two, Ash has recently managed to discover how to use aura by himself, and he and Goh arrive in Galar a week early, with Chloe and Dawn along (Chloe ran into Dawn before Ash and Goh left, and Dawn decided to go earlier than in canon, and convinced Chloe to join her), because Professor Ceries hopes to find out if there is a possible reaction if a human who can use aura tries combining it with Dynamaxing.
Chairman Rose and Oleana show up, and try to take revenge for ruining their previous plot, and disrupt Ash's aura as he's testing it, causing a ball of aura to grow in the same style as a Dynamax, then explode, washing over the Galar region, affecting every girl with romantic feelings Ash. These girls become Ash's love slaves and seek him out. They also lose any romantic feelings for other guys (with Chloe having previously had feelings for both boys, which she was conflicted about), and can sense his presence anywhere. Serena and Lillie were both in Galar separately, Lillie on a family trip, and Serena having made a brief stop in Galar for a performance, before planning to head back to Hoenn to meet up with May.
Chloe, Dawn, Iris, Serena, Lillie, and Bea must be among the initial girls enslaved, but others can be as well, and it is possible for other girls who love Ash too if they are close enough for long enough. The girls will have no issue sharing their master with other girls, as long as they like the girls.