• How can I get my 5120x1440 monitor working ?
    As of macOS 10.15.5, the Intel video driver that Apple provides for any Intel GPU before the 10th generation isn't able to manage any resolution above 4096 on a single video output lane.

    As such, there's no way to make such monitors work on 13" MacBook (Pro or Air) before the 2020 models (using Intel GPU).
    The only Macs supporting this monitor are the Macs using AMD GPUs (MacBook Pro 15", 16") and the 13" MacBook (Pro and Air) beginning with the 2020 Models (but again, only the ones using the latest 10th generation Intel GPU).

    The only way to get this working on any other 13" MacBook (Pro or Air) would eventually to use an external GPU.
    That's a limitation in the video driver provided by Apple, so it's not possible to change this with SwitchResX.

    This might change with a future macOS revision, but I wouldn't bet on this.