I don't use the .NET SDK, but using JavaScript (and standard REST APIs) and/or PowerShell (using REST APIs and/or the Partner Center PS Module) works fine to connect to the Partner Center.
You do have limited API functionality with an Indirect/Tier2 CSP Partner Tenant, you get full functionality only if you are a Direct/Tier1 Partner CSP.
The link to "Enable API access" you included works fine - it's just about registering your AAD/Entra App. You must use an account with Global Admin AND AdminAgent privileges. Works for your regular/primary account and Sandbox (I've set up and used the Sandbox in the past, but don't use it really any more), but the steps are current.
Authentication details are on this page:
https://learn.microsoft.com/en-us/partner-center/developer/partner-center-authenticationFor delegated admin access we mainly use the Native App + User access -- for the Secure App Model (not the Web App-only auth), but it will depend on what you're doing. You'll need to get a Refresh Token (and renew it at least every 90 days) then redeem it for an Access Token (good for 60 mins) and use that as the Bearer token (unless the .NET SDK takes care of all this for you?)
--Saul