I am maybe planning on continuing after LMoP with a custom campaign with some inspiration from some other books set in and around phandelver such as storm kings thunder.

As of now my players characters little to no backstory so I was thinking of letting them write a backstory with the help of the forgotten realms lore/locations and try to write a story around what they give me as backstories.

Any tips for starting a custom campaign as a first time dm?

Storm kings is perfect for this:  It's great if you are running a game in the north since it has info on locations of the entire region.  It has small blurbs and adventure hooks for tons of locations and is a great tool for making your own thing in this region.

Princes of the Apocalypse and Tyranny of Dragons also runs around that area.

You can steal from:
Dragon of icespire - It's set in Phandalin, but you'd have to adjust encounters since its lv 1-5

Acquisitions Inc. Orrery of the Wanderer-  Has a section in Phandalin that occuers after the events of LMoP, along with another run though the basement of Tressendar with different encounters.  It also wanders around this general area from Waterdeep up to luskan so you might be able to repurpose some of the material.

You could do some research on Waterdeep or get Waterdeep Dragon Heist. Waterdeep in terms of geography is not too far from Phandalin. You could even keep going southeast to Baldur's Gate, have a few quests there, then continue southwest to Candlekeep. However, if you're a new DM, this complicated layout may not suit you. You could re-flavour some of the Dragon of Icespire Peak adventures and fit them into the campaign, and then end it in Waterdeep.