I think you can make a twist of the Gen 3 idea that the players dad is a really strong Gym leader and they make their way to him
Just in this case the dad is Ash and he’s champion level so would have to scale for that

If you don’t want to make a long story I think you can do 2-3 regions to reach that level kinda like how Ash is on “E4” level at the end of battle frontier but just make the mc have a better start than he did so they can be on his level
If long story then you can make it 4-6 regions

Partner wise I think it would have to be a starter that has a chance to go further or have some uniqueness, like the mega starters (Gen 1 & 3), Z moves (Gen 7, Pikachu and Eevee), DMax (Gen 8, P&E), unique forms (Greninja, Decidueye, Typhlosion, Samurott)
Like personally I think Mega’s are the more interesting for story purposes so something like Mudkip works
Also if you have some idea in mind for them like your flying type trainer idea then make sure they pick Charmander or Rowlet

I think B plots helps it from being monotonous but it doesn’t always have to be an evil team I think maybe just a mystery can be enticing like finding a legendary

Companions, should go for the normal three person group route so someone to fill the friend role and mentor role,
Like you can have the kid of Brockx___ be a friend and new character be a mentor

For the mother I think it really depends who you decide the mother is, like if you pick Misty or Iris you can have them be a gym leader obstacle,Dawn or Serena can have the famous mom problem, someone like May could just be a teacher, Lillie a stay at home mom, etc
You can have the mom be pure encouragement like Delia or play a role in their story like Johanna or the XY parents

For Pokémon I think the MC should take a Paul route where he rotated between like 12 Pokémon, the number can adjust but I don’t think having a new team every region like Ash will work as well as it would just be a copy of him and this way it could be more believable that the character is strong as they keep training the same Pokémon

Depending on when your write it gens 1-8 are fine, if you plan to use Gen 9 either be patient with Pokémon and such news or just accept that you will have to keep Fuecoco in its base stage for awhile lol that or go full oc and make fakemon

Oh and to maybe solve the ending question, you already have it, defeating Ash, journey ends by defeating Ash
It's not something vague like being #1 or being a Pokemon Master, therefore they beat Ash the story ends