Advice

As you could probably tell I'm no expert at the game but I did play my previous account for three years so... yeah, take this advice with a grain of salt. But it might help you...?

• For energy, I use leifs ONLY DURING INCREASED DROP EVENTS. I have so many equipment materials (not upgrading mats though) so I really don't use Leifs on Hunts if Raid is running. I would if the upgrade materials dropped more frequently... and I use Leifs during increased rune drop events when I need to. Most of them are spent on grinding for catalysts now that the story mode system has completely changed.
• For the new player's selective summon with a free 5 star hero, I HIGHLY recommend Vildred for PVE farming. If for some reason you don't want him (or are planning to get Arbiter Vildred for the Moonlight Summon I suppose?), Iseria is also a very good pick. Destina is the hero that everyone recommends now since she revives on her S3.
• Speaking of the Moonlight Summon, I know that everyone recommends Specter Tenebria but I went for Dark Corvus since he's a bro a great tank for PVP. Honestly just choose who you want. Arbiter Vildred, like I said, is good for PVP and PVE and Ruele is a wonderful healer with invincibility.
• If you want to pull on a banner, summon away. Don't let the current meta stop you from enjoying the game! I would recommend looking up the character's role in the game so that you could utilize them better first. AS a general rule of thumb it's not recommended to pull on any regular RBG (Red-Blue-Green) unit banner unless it's a limited unit. You'll get all of them eventually. Hopefully.
• See all those speed sets listed for the characters? Yeah, you're gonna farm Wyvern hunts a LOT. Angelica, Sigret, and Furious are all free units that you'll get eventually and Muwi is a three star hero so hopefully you'll get him sooner than later. Alexa is also a good choice for Wyvern hunts, no need to complete her speciality change if you want to use her.
• Recommended side stories to complete if you haven't summoned the character yet: TAMARINNE, Vildred, Iseria, Mort, Destina. I added Mort because he's extremely valuble in PVP but for PVE he's outshined by a lot of units. Tamarinne is also a free unit and you can get her connection quests when you complete one of the last stages in Chapter Two. If you're behind on the story mode but would like to use her for other PVE content I would complete her side story ASAP. Also, the banner is not a 100% guarantee summon, it's just an increased rate up banner that lasts for a week for whatever character you choose. You need four "books" to open up said banner.
• As I previously wrote you should complete Tamarinne's friendship connection quests as soon as you could as well as Angelica and Furious'. I would also focus on Lorina if you haven't summoned her in the Moonlight Summon because her speciality change is very good but very grindy to get to.
• For my Chapter 3 selective summon I chose a set with Choux but Politis is widely reguarded as the best 5* hero pick. Senya needs her artifact to compete in the current PVP meta but she's a good choice too.
• DO MONTMORANCY'S SPECIALITY CHANGE AS SOON AS POSSIBLE. Ras' too but that happens at the end of Chapter Two so you'll have to wait a while. Lorina is also a worthwhile character to focus on as well.