Xcrun missing, installing Xcode Command Line Tools not fixing issue
Okay, After a full day of attempts, re-installing homebrew and all... found a solution which I should have tried a lot earlier in all honesty. Leaving it here if anyone else has the same problem in the future:

Doing xcode-select --install showed everything as being installed correctly but I just couldn't confirm that anywhere. I downloaded the CLT as a pkg from the apple developer website (https://developer.apple.com/download/more/) and installed it the old fashioned way. CLT is now installed and homebrew is detecting it just fine.

Brew Config now: 

HOMEBREW_VERSION: 1.5.14
ORIGIN: https://github.com/Homebrew/brew
HEAD: 7fd6210127f088b6ee8708a1d7f4ec2df3fc5bb4
Last commit: 6 days ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 1f9ba958e21dce9673b932cfc1f55dd155f0df69
Core tap last commit: 31 hours ago
HOMEBREW_PREFIX: /usr/local
CPU: octa-core 64-bit haswell
Homebrew Ruby: 2.3.3 =&gt; /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Clang: 9.1 build 902
Git: 2.8.1 =&gt; /usr/local/bin/git
Curl: 7.54.0 =&gt; /usr/bin/curl
Java: 1.8.0_77
macOS: 10.13.3-x86_64
CLT: 9.3.0.0.1.1521514116
Xcode: N/A
XQuartz: N/A


Thanks Nimesh for helping me!