As your flights are on a single ticket, you will be protected in the event you miss your connection at JFK.

Immigration at JFK T8 has improved significantly with the introduction of automated passport controls - visitors from Visa Waiver Program countries entering the US with a valid ESTA can use the APC lanes providing they have previously entered the US on at least one occasion after 2008. So if you qualify for APC, it shouldn’t take longer than 20 minutes to clear immigration.

On your return, as you correctly say, it is a simple matter of checking in your baggage at LAX and collecting them at HEL