Can you buy Shinkansen Limited Express tickets only?

Hi everyone, I wonder if anyone can help me with this.

We (4 adults, 1 child) will be travelling from Odawara to Shin-Osaka at the end of the month. One of us will be returning earlier than the rest, so I bought the 5 outbound Limited Express tickets and 5 Basic Fare tickets for the round trip between Odawara and Osaka City Zone, with the intention of buying the return Limited Express tickets separately (1 for the 31st, the rest for the 3rd).

However, I can't seem to find an option to buy just the Limited Express tickets on either the JR sites or on Klook. There's only the whole Basic Fare + Limited Express ticket.

Is there any way to buy just the Limited Express tickets online? Or do I have to buy the Basic Fare as well? I was under the impression that I already bought the Basic Fare. Maybe I'm missing something. This is my first time buying Shinkansen tickets and it's a bit confusing. I think I might have to cancel my current set of tickets and and treat it as two separate trips (4 people and 1 person).

Thanks in advance!