Hi Fristprime 
First off, yes, you can lock your character’s exp gain at an earlier level. To do so, you need to speak with Slahtz in Orgrimmar or Behsten in Stormwind.
Last I heard this worked with Chromie time as well, so you’ll be able to remain with her campaigns indefinitely.
Keep in mind however, that level 50 usually offers you the final step towards fully rounding out your character’s playstyle through the last row of talents. Some rotations rely on having this final talent unlocked, so a character may feel a little clunky at 49.
I wouldn’t be aware of any guides for starting quests of each expansion out there, but I’ve always found the storyline overview on wowpedia to be a pretty helpful start with this as it links each quest to it’s wowhead article, where comments can be really helpful for troubleshooting.
The problem with these breadcrumb quest is both phasing as well as finding up-to-date information. Sometimes, some of the quests aren’t available anymore since they were removed in previous patches, or the phase of the world you’re currently in makes it so some quests aren’t available to you at the moment (BfA has wreaked absolute havoc here, since a lot of it’s questlines bring you back to the old world, changing the phasing there). A lot of that information isn’t readily available, so it can sometimes come down to a case-by-case basis to resolve if any of the quests aren’t available for you.
That being said, here’s a list I tried to put together (take with a pinch of salt, I didn’t verify any of these in-game or anything). I’ll be listing out both Horde “(H)” and Alliance “(A)” quests here, even though I’m assuming you need the Horde side of things:
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Vanilla/Classic - No starting quest since there’s no main storyline, each zone has their own separate narrative
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Burning Crusade - Intro quest chain has been removed, now starts with (H) “Eradicate the Burning Legion”/(A) “The Legion Reborn” for the Hellfire Peninsula quest chains.
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Wrath of the Lich King - As you mentioned, the respective questlines start in either Howling Fjord (H) “The New Plague”/ (A) “Enlistment Day” or Borean Tundra (H) “Hellscream’s Vigil”/(A) “Hell Has Frozen Over…”.
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Cataclysm - From here on things get a little muddy. We know some of the quests have been removed, but it depends on what character you’re trying this on to see which one’s will be available. I’ll be linking the earliest possible quests here, but know that if it isn’t available for whatever reason, just go through and check the next quest in the chain. There’s 2 zones you can start in, Vashj’ir with (H) “Call of Duty”/(A) “Call of Duty” or Mount Hyjal with (H&A) “As Hyjal Burns”.
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Mists of Pandaria - Starts at (H) “The Art of War”/(A) “The King’s Command”; these chains are also notoriously unreliable in which ones are available and which not, so may need some breadcrumb finding by following along on the chain.
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Warlords of Draenor - Starts with (H) “Attack of the Iron Horde”/(A) “Attack of the Iron Horde”; if the quest isn’t available, you may be in the wrong phase of the zone, so speak to Zidormi to make sure you’re in the “modern” version.
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Legion - Starts with (H) “Legion: The Legion returns”/(A) “Legion: The Legion Returns”
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Battle for Azeroth - Starts with (H) “Battle for Azeroth: Mission Statement”/(A) “Tides of War”
I hope this helps, but before I close this out, a little personal tip here: I know doing everything on one character seems enticing for completionist reasons, but just know that this can turn into a bit of a slog. You’re much better off creating one character per expansion, and leveling them through each to experience the story. Loremaster achievements for each expansion are account-wide, so you won’t lose out on progress if you switch between characters for each expansion. You will also be able to experience many different playstyles and builds along the way, and may even want to create a character on the other faction (Alliance is much better anyways 
), and at the end of it all you’ll be left with a minimum of 7 level 50 character, giving you more options to go into Shadowlands.
Obviously this is just a recommendation, but I figured I’d add it here, since I’ve seen it mentioned before that doing everything on one character can be a bit of a grind.
Alright, that’s it from me, sorry for the wall of text, but I hope this is helpful! Shoutout if there’s any questions 