Seat reservations are usually same price, sometimes even higher with first class. For a line without mandatory reservations you might have higher chance to find a seat without reservation in 1st class, but beside that, one cant resoning buying a first class in order to not need to buy seat reservations.

Maybe youd like to share with us your route for recommendations.

Some confusion here.

1st class regular passes need to be accompanied by paid for reservations on those trains/companies that require them.

It is the Plus Pass (available in either 2nd or 1st class) that has included reservation fees.

https://plus.interrail.eu/en/interrail-plus-pass

You have to use the Interrail reservation service with these but most reservations should be with no extra charge.

Do you have a Plus Pass or a normal Pass?

The only country I know of, where first class passholders save the reservation fee is Norway.

Did you read anywhere on the Interrail website that with a 1st class pass the reservations would be free of charge? If so, then please let us know the URL because that's incorrect.

For reservations, it's best to use this guide:

https://www.seat61.com/interrail-and-eurail-reservations.htm