Saves and meeting points: Finish city quests before leaving and activate the main story checkmarkMinor spoilers
Leaving town, taking exams, and resolving story events update your save. Visit meeting points first, distinguish battle retries from chapter restarts, and replay the rest of a route after fixing missed content.
Original video: Fire Emblem: Fortune's Weave tips for avoiding mistakes and classes unlocked on each route | Bilibili creator Maoshu Jia de Sanhua
Finish city requests before heading out, confirm the green check at the main-story meeting point, and do not treat low-pass-rate exams as attempts you can undo by reloading. After playing two routes, I find these steps save the most repeated work from autosaves.
Leaving town and taking exams both trigger autosaves
Most of my mistakes across these two routes have involved autosaves. I would save before leaving town, intending to look around outside and then reload, or attempt an exam with a low pass rate just to try it. After the action, the game had already saved, and I could no longer return to the state I wanted.
At the start of each chapter, finish any quests you can turn in within the city. Leaving and returning takes time, so avoid an extra trip for something you could have handled immediately. Do not rely on luck with low exam pass rates, either. Both success and failure update the save.
Defeating the boss on a counterattack ends the map early
Dietrich has strong counterattacks. If the boss charges him, he may defeat them immediately and end the map.
If regular enemies have already flanked you and defeated an ally, or you still have unopened chests, meeting the victory condition leaves no time to finish your original plan. Clear regular enemies before engaging the boss where possible. Otherwise, move your other allies out of danger first.
Activate the green checkmark at the main story meeting point
For quests with a designated meeting point, visit it first and activate the green checkmark in the lower left before exploring elsewhere. When the date arrives, the game returns your team to the main story.
Do not plan to finish everything along the way before heading to the meeting point at the last moment. A chapter can contain over 100 turns. Miscalculating the schedule and restarting means repeating all that travel and work. Spending time visiting the location first is easier to accept than missing the meeting.
Choose carefully between retrying a battle and restarting a chapter
If your starting positions are wrong and you want to redeploy, read the entire menu first. The options to restart the current match and restart the chapter sit close together. Choosing the wrong one sends a whole chapter's preparation back to the beginning.
To replay only the battle, select the battle retry option. To redo the chapter's free actions, select the chapter restart option.
Fixing an early chapter still means replaying the rest of the route
Some paralogues have very short availability windows. If you later discover that you missed recruiting someone, you need to return to the relevant chapter. The sooner you catch it, the less you need to replay.
Restarting an early chapter and completing that small section does not immediately update your later progress. You still need to finish the following route sections to bring the new development and recruitment results into Parts 2 and 3. Returning to a chapter gives you a way to fix mistakes, at the cost of replaying the long stretch afterward.
Unlock special classes on the required routes first
Some classes require quests on specific routes. Once unlocked on another route, they can also be used on the current one. If you can see 
Wing Soldier but cannot find the 
Dragoon you want later, do not simply wait for it in the current exam list.
For Dancer, 
Caladrius, or special classes from other routes, advance the relevant route to its unlock quest first. Complete it, then return to planning unit development. That way, the class will already be available when your unit reaches the required level.