I have the same bike OP and it looks like you've narrowed it down correctly. You need a 700c x 135mm mountain bike rear wheel that takes an 11 speed cassette (specifically Shimano 105 "road" components). You can't use a wider wheel and you don't want to use spacers.

The stock Axis 2.0 wheels are a little bit heavy but they're not particularly slow or uncomfortable. In those regards you'll get much more reward from changing your tyres and/or pressures. And tyres are a lot less expensive than wheels.