It's a massive bugbear for me and I'm sure many others.. having to build a pipeline once before you can 'Build with Parameters'.

It could be as simple as checking out just the jenkinsfile(s) during the repository scan and load the parameters before the first build (obviously as an option to enable it as it would slow down the repository scan).

Alternatively, give us a 'load/refresh parameters' button that starts the pipeline but only until it has got the parameters and then stops by itself before doing any build steps.