That's been known about for at least 8 or 9 years, but it's been obsoleted by other methods a few times.
As with most runs, it's best to learn hands on in my opinion. Make sure you have most basics down from other tutorials and apply what you learn there to any run you see. Practice is definitely the most important part no matter what. I'd definitely make sure you're familiar with how long to ground yourself to collect a berry and which ones you definitely don't want to lose by dying after collecting. I find it wise to break it into chunks, so full clear ILs might benefit you greatly. If there is a tutorial out there, I'm sure it would also be useful.
The game is a GBA game, the listed platforms are just the ways you can play them. As for which is best, I'm not sure. I know there was a load comparison for a Kirby GBA game, and there were differences that made WiiUVC faster than DS and DS faster than GBA. I can't say it works the same for this.
There are plenty of PS emu runs listed on the board already! :)
JoytoKey, probably. I'm not sure, though!
Luckily, the link you provided is from a runner that uses input display. It could be worth taking a closer look at it to study and figure out the details that way.
An IL board is a much different concept than a full-game category split.
Sit at a bonfire and look through the options.
I don't want to name names, but @goobert what were you thinking when you approved @RadicalMacaroni 's speedrun of Bowl%?
Run in question: https://www.speedrun.com/chex_quest_hd/run/z05p7o9m
It is clear to anybody with 20/160 vision or better that the cereal being scarfed down is CHEERIOS. The light, fluffy nature of the cereal gives a competitive advantage over anybody following the rules of the category, which state you must eat CHEX cereal. If you must keep a TAS up on the leaderboards, please make a separate category so we can have fair competition in the categories that matter. Thank you.
98% is all Trophies, Gems, and Crystals through Oxide, not relics. :)
The Warp Room 1-3 speedruns were how the google doc boards were structured before there were hardly any Any% speedruns. Runs that don't aim to reach the credits are having less and less of a presence in speedrunning, and the appeal of these categories in the past was to shorten what used to be a 3.5 hour speedrun. I've personally done a race of your first idea, and it turns out to be 98%. I don't feel it's fit for a leaderboard.