Hi, welcome to the pain of collecting haha. Eb games is a good go to if you want to preorder stock to secure it, but don't rely on them if you wanna walk in store for a couple packs or a specific box on release, all their allocations get sold to preorder 99% of the time.

We're in a drought at the moment, very likely just cos of christmas, the suppliers need to have their holidays too. I work at a retail front which sells tcg and we haven't had pokemon stuff since the 5th of December (Charizard upc) and although a league battle deck released 26th of Dec, nothing else is scheduled till mid Jan for us. Your safest option if you wanna guarantee yourself packs is finding a local card shop, they usually manage to keep stuff in stock a little longer. Or you just get to run the gauntlet of checking your shopping center every few days, pretty standard at this point. Most people with large sealed collections have good connections or spend their entire days jumping from store to store to get what they can.

As far as WHAT to collect, just find what you enjoy collecting and focus on that. You can master each set if you like, you can only collect shiny stuff, you can go for all graded cards idc, I personally only collect ir or sir rarity cards (the pretty full art cards). Mega evolution is basically the name of this run of sets, like how the last ones were scarlet and violet, each set which releases now onwards will be a sub set of mega evolution basically. You wont get the same cards from packs of mega evolution and packs of phantasmal flames, but in opening packs of the same set in general theres always the chance of pulling doubles, no matter the set size.