Ouch, that fleet is not at all the best way to start out.
Don't mean to be negative but Endurance is hands down the worst capital ship.
And as your discovering the Zeta challenge requires Executrix.
This is why I preach DS ships first & start on Kylo-U long before hitting L60.
FYI. The Zeta challenge is a royal pain in the arse & I don't recall beating it before L85 myself.
I ended up having much of my fleet at 7* before I could pull off a win.
So, now for the long recovery.
Stop investing in ANY LS ship at this time.
Red/Cantina Energy = Farm Kylo-U & his Ship from same node. 10 Energy.
You might as well just plan on taking him straight to 7* now.
G-War/Green Coins = Ships... Geos & Imp TIE.
Purple/Fleet Coins = Sun Fac, All 3 Geo Ships, or Slave-1, TIE-Adv-X1. Nothing Else.
If you can afford a double/triple refresh for the next 5 days on Kylo-U you'll get a bunch of TIE Bomber Shards.
So you'll have a head start in 6 weeks or so when they stick it in a Node somewhere.
Just keep farming the DS ships till you can pass that challenge & they will be better than your GR ships in arena in a short while.
I started early last year before Brood Alpha so I didn't have Geo's.
I went TIE Silencer, TIE Advanced, TIE Fighter, FO-TIE Fighter & SLAVE-1.
I still have these 5 & only added 3 more BH ships to my DS fleet & use them as back up in TW/GAC.
Knowing how good Geo's are now, & how long the Slave-1 & Vader farms are, I would swap out those 2 & FO-TIE for the 3 Geo Ships since they have nice Synergy & fast Fleet/Cantina farms.
For you, if you can get the TIE Bomber open then you don't even need TIE Fighter and can go Geo's + KylU + Bomber, which will make for an interesting fleet I think.
Good Luck.