Everyone's already given some really good tips (even stuff I didn't know, like no SP gain without rings!), but I'll add in some. These are definitely not necessary but are good for certain play-styles:
If you want to get even more upgrade materials for your weapons, put a bunch of dogs on your farm in the Somniel. Dogs give you ores. Other animals give you food items. After adopting them you have to manually go to the farm and choose which animals will be in the pasture.
Unless you're interested in adopting every animal, it's probably not good to invest a lot in donating to other countries. After certain points, it gets very expensive. First level is good if you want cooking ingredients/better skirmish rewards.
There are ways to farm for certain bond rings in the gacha without wasting too many bond fragments. If you're interested, there are several tutorials on youtube (which also tell you about the bond rings with the best S skills). It's better to save fragments for unlocking and inheriting emblem ring skills.
ALSO...if you ever intend to use the DLC emblems on another run, make sure to do all the divine paralogues (where you get the DLC emblems) BEFORE completing the fell xenologue (the new story). The enemies on DLC emblem maps will be jacked up to much higher levels once you complete the fell xenologue, so unless you want a challenge, be sure to get the emblems first. You receive the DLC characters at level 20, so it's best not to finish the xenologue before chapter 13 of the main story.