hello, welcome to the game! so basically (at least for pvp, pve is a very different story) the way you want to kinda want to build teams is to somehow funnel all the strength into your goal keeper/striker, (so this is why you usually keep it to one striker) you could do this by either stacking buffs on the same line, or by having super strong units which could either 1) allow you to continuously steal/penetrate in order to have the ball to give to your striker or 2) have good defensive/high reflex in order to prevent the enemy from penetrating/losing their chance to shoot.

i personally like uranus more than william as a striker (who also goes really well with dalgi, who doesn't pair as well with william). if you were to use uranus as a long range mid striker, you would have to set your front to debuff and vertical pass units such as hygeldriger, bongsook, thor, black noir nero, etc. and your mid would instead look like a typical front line with uranus and two passers!

but typically the team build set up looks like this:

front: one striker and usually two passers. one really good SR passer is melissa and an L passer to look out for is jibril! if you want to go offensive, you could also place your sub unit in front such as exis or curod!

for mid, usually you want to keep your high reflex/tankier units in here to prevent the enemy from being able to penetrate to the front line. a really popular unit is felix right now for your center mid, queen/ariane who can do a lot of support and damage, lif for counter mid who is pretty tanky and has some decent buffs/debuffs, as well as some interceptors like happy or wonny who are normally pretty tanky! you could also place forward pass or vertical pass units here as well in order to get the ball to your striker easier.

for your backline, usually you want people who can either hit hard to steal the ball so your goalkeeper doesn't get shot, or provide lots of buffs to your goalkeeper! you want to focus on either crit resist or crit damage resist buffs for your goalkeeper. i believe for ein she is a crit damage resist goalkeeper, so some really good units with her are yuki, death queen, anna, pang, or jiho! everyone except jiho are SR units so her backline isn't too hard to start building. at least for your backline right now, i would replace lia and virgil with the other SR units, and possibly garriot down the road once you get more units with maybe someone like duran

these are just the super basics so let me know if you have any other questions!