GDPR data subject rights cannot be waived. Data protection is a fundamental right, based on Art 8(1) of the Charter and Art 16(1) TFEU. However, GDPR balances this right to data protection against other rights, and explains modalities around how this right is guaranteed.
The GDPR does allow data subjects to agree to processing activities that might be against their interests, by giving consent. However, consent must be freely given, specific, and informed, a high bar intended to protect data subjects. Explicit consent can be used to authorize certain activities that would otherwise be prohibited, e.g. processing health data, or transferring data into countries without an adequate level of data protection. “Do whatever you want” carte blanche permission cannot be valid GDPR consent, because it is not sufficiently specific.
The data subject rights in Arts 12 to 22 GDPR cannot be waived. The rights are necessary to hold data controllers accountable. These rights already include limitations to protect data controllers from data subjects that exercise these rights abusively. There is no mechanism that would make it possible to make contracts requiring the non-exercise of these rights, that would go against their purpose.
However, data subjects are not required to invoke the Art 12–22 data subject rights. They can voluntarily not exercise them. This is in contrast to the fundamental GDPR principles like lawfulness or accountability, that data controllers must always fulfil, regardless of whether a data subject explicitly invokes them.
You suggested this phrasing in a request for information:
I don’t intend to compel you to prepare a SAR response for me but if it happened to be really convenient and you felt like doing it then it would be really helpful to me to have this information.
That does not sound like a valid Art 15 data subject access request, because it clearly does not try to invoke the right to access. But it's not a general waiver of that right – the right to access could still be properly invoked at any later time.