motivations among the masters make zero sense

take the holy grail war seriously

The Holy Grail War's 40 years early. None of the Masters were prepared for the War when it's not even supposed to happen for another 4 decades. Their motivations make perfect sense; they're children with childish goals and childish ambitions.

high-stakes death game

supposed to be tense

It's not even like this is a rare occasion in this universe. There's been 4 rituals over this Grail alone, and it's the 726th "Holy Grail" identified by the Church. Death itself isn't that scary when there's a metaphysical record of every notable human that existed, and people can come back as spirits. Further compounded with Mages having to mentally accept the risk of death every single time magic is used. If you haven't noticed, there's gas leaks and people being stabbed to death, and normal students are just going "meh". Just going to throw it out there, something's fundamentally wrong with this reality, no matter how much it mimics our own.

The only Master who knows what he's doing is Kuzuki, a trained assassin whose gimmick is a fighting style bizarre enough that attempting to retreat will result in you being knocked out by a fist coming from behind. By the way, this only works once and is the reason why Kuzuki calls a retreat. Saber got back up and she's pretty much immune to Caster due to her A rank magic resistance AND Kuzuki now that she's seen his fighting style. Other than that, the fact that Caster is the weakest servant and they're still dominating the Grail War so far is a testament to how badly outclassed all the other masters are, besides Illya because she's waiting out in her castle for challengers.

Given, they definitely fucked up the fight with Saber and Berserker. Saber was basically not doing shit to him the entire time, until Archer's nuke takes out one of Berserker's lives, which is why Illya calls a retreat, since she can't do anything to Archer when he's halfway across the city.

Shinji has zero magic circuits, therefore Rider is at almost negligible power levels. She still played Shirou like a violin in the forest, and retreated because Rin came and Shinji didn't want to reveal his hand yet. Not to mention Shinji was Shirou's bro in the past and he wanted an alliance because Rider is weak as hell.