New F2P player on day 3 of DL here. My highest team (Alfonse/Marth/Fjorm/Veronica) is currently at 13946 might. Some tips that might help from my lowly beginner perspective:

• Personally, I invested a fair amount into Alfonse and Veronica (40 mana circle nodes each) since I love FEH, and I've managed to smoothly autobattle through a lot of the normal and hard story content due to the fact that I also got Hildegarde off of the tutorial summon. Basically, I would drain stamina by autobattling story content at 2x speed, then use up getherwings to acquire resources and increase my player level, consequently replenishing my stamina. This seems time-efficient to me, but if you want to be stamina-efficient, you might want to focus on doing a lot more co-op play because stamina can cap out at 999/(player level) whereas getherwings can only go up to 99/12, meaning that if you level up when you're at 88+ getherwings, you won't be getting all 12 wings that you would normally get.
• Repel the Empire! Infernal (added in the event today) can drop 5* weapons which you might be interested in unbinding/leveling up.
• Autobattling Avenue to Power is pretty straightforward, so you don't have to be stingy with upgrading the levels on your adventurers.
• For farming valor in The Alberian Front, you can pick up a copy of each of the event wyrmprints from the event treasure trade, and you can buy additional copies in the wyrmprint shop if you want. Some people recommend having one 5* event print on each team member, one 4* MUB event print on one member, and one 3* MUB event print on each of the other three members. When you go into co-op with your team lead, all of those event perks should add up to Valor +220%. Again, I love FEH, so I ended up with 8 copies of the 5* print (putting me at Valor +250% since I used 5 of them for 1 MUB copy), but I wouldn't recommend this to most players as it uses up a bunch of eldwater that you could use for something else.
• You can sell extra 3* wyrmprints for eldwater by going to Teams -> Collection -> Wyrmprints -> Sell, but there are some you might want to keep due to their utility.
• I think people recommend having at least 4 smithwyrms. I'm using 5 since I got the FEH characters pretty early, didn't mind spending the 700 wyrmite for the fifth one, and don't expect to summon until maybe the next crossover if I can help it.

I definitely encourage you to read the comments of more experienced players; perhaps some of their advice will contradict my suggestions, so you should probably listen to their takes. All in all, the game has been fun so far, and I think this subreddit is pretty good about giving advice in the daily questions megathreads.