I usually stick to either single female leads or a limited number of them. While there are a lot of beautiful characters in Pokemon, the female leads in this type of novel are really just there to contribute to the plot.
Writing in today's world is different from the past. Even a simple kiss has to be carefully written. So I don't think it's very meaningful. If it were an original protagonist, I'd definitely go with the Cynthia (Hibiki) route. If it were Ash, Serena would be the focus.
Of course, every female protagonist, except for Iris, I think, has her own dedicated ship with Ash. But if we analyze it, the number of female characters who clearly show affection for Ash isn't many.
First, Misty is definitely in love with him. You could see it during the Orange Islands arc. There's also that rescue story, though I don't know if it's available anymore.
It's about Misty's story, where a guy has a crush on her and gets rejected. When the guy asks Misty if she likes someone, she blushes and denies it, but Psyduck is grinning mischievously next to her.
She's the first female lead to appear and the one who has traveled the most with Ash. So even though she doesn't appear much now, the Ash-Misty ship still has a strong following.
Then there's Ash-May. I think there are fans who have asked May about this ship, but she's also shy. But for some reason, I feel like her shyness isn't the kind that comes from being caught in a crush, but rather, simply being teased and embarrassed.
This ship feels like something fans have mostly created themselves, giving rise to the Ash-May couple. They're more like good friends.
Next is Ash-Dawn. This pairing is the most popular according to a reputable CP survey in Japan. Ash-Dawn has even surpassed Ash-Serena. Maybe it's due to their personalities.
Dawn is a bit more gentle than the previous two female leads. It seems Japanese people are more into this type. But from the ending, it looks like Dawn is quite calm about the separation.
Then there's Iris. Honestly, this pairing has barely any popularity in either the international or domestic communities. But I'm not sure what kind of aesthetic or taste the Japanese have. This ship actually ranked pretty high in their surveys.
It's like how we're annoyed by the character Cilan, but Japanese people don't seem to dislike him that much.
Ash and Iris just don't have the chemistry, not even compared to the other pairings. And their appearances don't fit the aesthetic of most fans either (though her champion design is pretty good).
They feel more like buddies, so it's basically impossible to consider them.
Personally, I like Serena the most. It's partly because of how well the Kalos arc was done, and partly because I like Serena herself.
She's gentle like Dawn, plus there's that heartwarming moment when she recognizes Ash from when they were little kids after they meet again as adults. And I feel like they have the most chemistry.
Even though Ash is still his usual oblivious self in the plot, he's not as blunt towards other female characters. For example, he doesn't act the same way towards Serena when she wants to shop or participate in a performance contest.
If I remember correctly, Ash has argued with other female leads over similar things. Like in Rainbow City, Misty wanted to buy perfume, but Ash was solely focused on the gym battle.
May and Dawn also argued over whether to do the gym battle first or the contest first. Iris pretty much argued with him every day. Serena and Ash argued too, but I don't feel it was actually arguing. It was more like a display of affection, during the time when Ash was down after losing a gym battle.
And all the other female leads actively joined Ash on his journey, but Serena was invited by Ash. He even took the initiative to suggest going to the city where the performance contest is held first, then challenging the gym.
To sum it up, this is just my personal preference. I won't have too many female leads, but they'll all appear. You don't need to worry about it. If you like a certain pairing, you can just ship them when they appear.
You can even ship characters besides the female leads. If it wasn't Ash, my first female lead would be Cynthia. If you want to see Cynthia, I can write that too.
Don't worry about age either. If you look at Japanese anime fanfiction, you'll understand their view on love. Age gaps are completely normal there.
Have you seen the old series, Cardcaptor Sakura? In it, the reincarnation of Clow Reed is an elementary school student, but his partner is a female teacher. Even though Clow Reed is still alive, he should be hundreds of years old.
But after his reincarnation, he's in the body of an elementary school kid. It's basically the same as Ash, an adult soul in a 10-year-old body.
And there are many strange pairings in Cardcaptor Sakura. Like one of Sakura's classmates is in love with their elementary school teacher, and the teacher accepts her feelings.
A bunch of elementary school students are in love. It didn't seem like anything back then, but I started thinking about it when I grew up. Isn't that underage dating?
But later, I learned that there's no concept of underage dating in Japan. If you ask a teacher about it, they'll be surprised because they don't even know the term.
So the bottom line is, ship whomever you like, and you can just enjoy it when it appears. The female characters will all have a good relationship with the protagonist, but who ends up together in the end isn't something you need to worry about.