Frankly if you’re f2p or only spend a little, just focus on getting all the good supports and borrow ST servants from a friend. For example, if you have Waver and Skadi, borrowing an NP5 Scathach or Okita would likely give you better performance than having NP1 of your own, while borrowing a Skadi.
Don’t worry too much about developing your ST lineup, you’ll eventually get there from a combination of spooks, welfares, or high NP low rarity units. If you care about sq investment return the first thing you should develop is your support lineup. After that you can aim for farming units (AoE NP) like Dantes, Zerkerlot, and later on Space Ishtar and Zerker Musashi.
If I have to pick one SSR servant that’d perform exceptionally well at NP1 for a new player, I’d choose Cu Alter though. My friend’s Cu Alter (with my Mash and Hans) carried my ass from Fuyuki to Solomon in 2-3 weeks or so. I started a couple of weeks before the first Merlin banner dropped and I made it to the Solomon tentacle raids (except Barbatos who was barba-toast within hours or so).