After New-IshSession ... -Protocol OpenApiWithOpenIdConnect, you can use $ishSession.OpenApiISH30 proxy to do REST calls over. Similar to doing SOAP calls like $ishSession.Application25.GetVersion(). These are the proxies used inside the ISHRemote cmdlets like Get-IshVersion to continue the example.
Current state of OpenApiISH30 is InfoShare API v3.0.0 (20240404). New functions were added which become accessible after a refresh the api spec json as initially done on #180
Remember that New-IshSession ... -Protocol OpenApiWithOpenIdConnect, so with the protocol is required to access these proxies.