SPA Page navigation metric

We have an SPA application with some pages that have client side routing implemented.

Before CSR was enabled, we use to monitor LCP as a key vital metric. However, since CSR was implemented, LCP is no longer a reported metric.

My question is which metric is best to track the performance improvements that are carried out on CSR pages - is it Speed Index or something else? We would like to track the performance improvements in pre-prod environment where we have tools like WPT/Speedcurve for Synthetic testing before moving to Live.

Any help on this is much appreciated.