I want to plan my team, but I don’t know how to translate where a Pokémon appears into how far in the game they show up since I don’t know the order we can get to areas.
I'll try my best to tell you where, using some help from the spreadsheet. Unless specified, you can find them right from the start without any barriers. Generally, first area's barrier is the dividing bridge, the third's barrier is the sea and the fourth's is wayward cave and the small section afterwards. I don't think the second has any barriers. Can be slightly inaccurate regarding the time you can first find them.
Snover Family: Last area, very common. You can't miss them
Abra family: First area, slightly hidden and full of overleveled mon for the area. Needs berry shenanigans due to it's running style.
Aipom family: Third area, there's literally an aipom hill.
Bergmite family: Last area.
Barboach family: Second area. Needs to catch them in the rivers but you don't need the third area's basculegion.
Shieldon, crandios, porygon, the original sneasel, magnemite and starters' family: Distortions.
Wurmple Family: First area. Extremely common.
Chansey Family: Spreadsheet says first area but i started to see chansey at the third area.
Bidoof family: First mon to catch.
Bonsly family: Second area.
Rufflet family: Last area, very common in lakes and such.
Bronzor family: fourth area, hard to miss. Requires some quest progress.
Budew family: second area, literally can't miss.
Buizel family: First area, hard to miss
Buneary family: First area, Near the arena. Requires some quest progress
Burmy family: Grass form first area, ground form second area, steel form third area. Jumps out from trees. Requires some quest progress( I think)
Carnivine : second area, very common.
Chatot: third area, right side.
Cherubi family: first area. Can't confirm because haven't personally seen it.
Chimchar family: Do will-o-wisp quest or wait for basculegion. If the former, first area; third area otherwise.
Chingling family: fourth area, hard to miss. Requires some quest progress
clefairy family: fourth area in a specific place, only appears at night i believe. No idea about cleffa. Requires some quest progress
Combee family: first area says the spreadsheet but I can't personally remember so no confirmation here. No quest progress for the second area though.
Croagunk family: second area. Requires some quest progress
Zubat family: first area. Requires some quest progress
Skourpi family: second area.
Drifloon family: first area at night.
Duskull family: third area, right side at night.
Eevee family: first area left side but only one and rare so you need a keen eye.
Elekid family: Fourth area, around the snowy northern part. Requires some quest progress
Voltorb family: fourth area, near the arena. Requires some quest progress
Piplup family: Third area, requires basculegion
Finneon family: Third area, requires basculegion
Snorunt family: fifth area, common.
Gible family: fourth area wayward cave.
Ralts family: second area right side, there's a ruin like thing.
Shellos family: first area right side, slightly hidden and full of overleveled mon for the area
Glameow family: third area, hard to miss.
Gligar family: fourth area. Requires some quest progress
Psyduck family: first area. Requires some quest progress
Geodude family: first area. Requires some quest progress
Goomy family: second area, requires either stantler shenanigans near the unreachable island or basculegion
Turtwig family: second area, very hidden. Go to the right end of the map, it's near a small pond surrounded by ursarings.
Growlithe family: Third area but stantler shenanigans are required to get to the top of the mountain.
Magikarp family: requires basculegion i believe.
Heracross: fourth area. Requires some quest progress
Hippopotas family: second area.
Murkrow family: second area at night.
Scyther family: first area near the arena. Requires some quest progress
Kricketot family: first area. Requires some quest progress
Lickitung family: second area.
Petilil family: second area near arena.
Riolu family: Quite hidden. The best clue i can tell you is to try to climb the mountains on the left and search for an alpha lucario. There is a riolu at the bottom of the mountain.
Shinx family: first area, can't miss.
Starly family: first area, can't miss.
Machop family: third area i believe.
Magby family: third area firespit island and thus requires basculegion.
Swinub family: last area, very common.
Mantyke family: Third area, requires basculegion
mime jr family: first area right side, slightly hidden.
misdreavus family: fourth area at night. Requires some quest progress
munchlax family: first area.
Vulpix family: third area.
Alolan vulpix family: quest fifth area from what i've heard.
Nosepass family: fourth area. Requires some quest progress
Remoraid family: Third area, requires basculegion
Onix family: fourth area. Requires some quest progress
Qwilfish family: Third area, requires basculegion
Tentacool family: Third area, requires basculegion
Pachirisu family: second area
Paras family: first area. Requires some quest progress
Pichu family: First area, hidden on the right side of the map
Ponyta family: first area.
Rhyhorn family: second area.
rotom: fourth area night. Requires some quest progress
Spheal family: third area
stunky family: second area
Hisuian sneasel family: fourth area. Requires some quest progress
teddiursa family: second area
spiritomb: i believe you need to finish the will o wisp quest.
tangela family: second area.
togepi family: second area
yanma family: second area.
stantler family: first area. requires some quest progress
Zorua family: fifth area, hidden and you need to go underground via a hole. (There are multiple of these)