I have an older version of Lightroom (v5.4) and want to upgrade to v5.7. I've heard this doesn't work well or at all on High Sierra. Has anyone had experience with this problem or do I just need to breakdown and purchase an upgrade. Thanks for any suggestions.
Lightroom 5 is an older product and we have tested running the latest versions of Lightroom Classic CC and Lightroom 6.x with macOS v10.13 (High Sierra).
You may refer this article for more info: https://www.lightroomqueen.com/lightroom-macos-high-sierra-compatibility/
I'd recommend that you upgrade to the latest version Lightroom Classic CC.
... or do I just need to breakdown and purchase an upgrade.
No need to breakdown.
Just purchase Lightroom 6.14 or a subscription to Lightroom Classic CC. NOTE: Lightroom 6.14 is unsupported software, there will never be any updates to it, and there is no support from Adobe.