Here is the complete request:

<soapenv:Envelope xmlns:xsd='<http://www.w3.org/2001/XMLSchema>' 
    xmlns:xsi='<http://www.w3.org/2001/XMLSchema-instance>' 
    xmlns:soapenv='<http://schemas.xmlsoap.org/soap/envelope/>'
    xmlns:platformCore='urn:<http://core_2020_2.platform.webservices.netsuite.com|core_2020_2.platform.webservices.netsuite.com>' 
    xmlns:platformMsgs='urn:<http://messages_2020_2.platform.webservices.netsuite.com|messages_2020_2.platform.webservices.netsuite.com>'>
    <soapenv:Header>
        <tokenPassport xsi:type="platformCore:TokenPassport">
            <account xsi:type="xsd:string">${NetId}</account>
            <consumerKey xsi:type="xsd:string">${consumerKey}</consumerKey>
            <token xsi:type="xsd:string">${tokenId}</token>
            <nonce xsi:type="xsd:string">${nonce}</nonce>
            <timestamp xsi:type="xsd:long">${timeStamp}</timestamp>
            <signature algorithm="HMAC_SHA256" xsi:type="platformCore:TokenPassportSignature">${signature}</signature>
        </tokenPassport>
    </soapenv:Header>
    <soapenv:Body>
        <search xmlns="urn:support_2020_2.lists.webservices.netsuite.com">
           <searchRecord xsi:type="q1:SupportCaseSearch" savedSearchId="734" />
        </search>
    </soapenv:Body>
</soapenv:Envelope>

SupportCaseSearchAdvanced looks right, but the q1 namespace looks undefined

Yeah, that was the issue. Thanks for your help @battk