macbook pro – How to know when battery is “failing” according to Apple?

I have a 2017 MacBook Pro without Touch Bar with 1090 battery cycles, 70% battery capacity and as condition it says “Replace Soon”. I bought the computer 2 and a half years ago so my warranty is no longer valid.

I contacted the only two apple service repair shops near me and if the battery is faulty they will charge me the equivalent of 250USD, but if they find no fault they will charge me 450USD. That’s outrageous, it’s almost twice the price if they find no fault in the battery because they will also charge me for the keyboard (they said the keyboard is on Apple if the battery is found to be faulty).

I can bring it in for a diagnostic for 40 bucks (that go towards the replacement if I go for it) but I can’t afford keyboard+battery, I can only afford to be charged for the battery (250 USD). I asked what is the test they do to know if the battery is failing, if it’s the condition in the system report or if its something else. They told me it’s something that can only be done at an Apples Diagnostic center.

If I take my computer they will have it for 2-5 business days and I can’t spare that time if the battery won’t be replaced. My computer is my work and I can only afford to be away from it to replace the battery for the 250USD. So, before I take it to be repaired, I need to know 100% that when they test it they will “find a fault with the battery”.

So, what do they do to test that? It’s obvious the store is not gonna tell me, but maybe I can find the info online. Would it be bad for my computer if I wait for the condition in the battery to be “Service Battery”? I am assuming if that message pops up is because it is for sure failing so if I can’t find a way to test the battery I might as well wait for that message if it warrantees that they will find a fault.

My computer shuts down randomly, it happened more often about a month ago. Sometimes the battery switches from 34% to 4% and then back to 34% once plugged into the wall. It doesn’t last more than 2-3 hours when it’s on battery.

Edit: I am on macOS Mojave