I did this a couple years ago (I think in 2023?), and processed to not touch a pokemon game for around 7-8 months. Genuinely burned myself out.
The Mew is an Aura Mew, Celebi from the Japanese Bonus Disk, Jirachi from Colosseum Bonus Disk.
The Johto starters at the end are all from Emerald, as the post-Dex completion gift, and I plan on at some point getting the trio from Mt. Battle.
Both Espeon and Umbreon are from the 10th Anniversary event.
The Plusle is formerly Duking's.
A large amount of the pokemon here are former Shadow Pokémon.
The Ho-Oh and Lugia are from Navel Rock, and I plan on eventually adding Mt.Battle Ho-Oh and XD001 Lugia.
The Latias is from Southern Island.
The Deoxys is from Birth Island.
And the Poliwag was found in FireRed in the pond in Fuchsia City after I realized I had forgot a fishing encounter there. Only shiny I found during all this.
I ended up playing through all 7 gen 3 games.
All ended up taking about a year, give or take a month or two.
Added at the end is the team I used in the run of Emerald.
by LoudDream2916