good advice. if doing this would slow OP down, use a turnkey service.

Okay, what happens if there are bad actors among my users who use my service to send Nigerian prince emails or boner pill ads?

How will that affect my relationship with Sendgrid?

Depends on the service, but all of these services will try to deal with reputation damage by a series of preemptive and reactive actions.

Preemptive being checking the emails before they go out for malicious links etc, and reactive being checking the deliverability (delivered, bounced) as well as optionally email actions (open, click, ubsubscribe, flag) and stopping the account from sending further emails if something bad is happening.

This last part "blocking the account" is what will save you from waking up one sunday morning to find out your domain has been added to all spam lists and no one wants your emails any more. -- This can be even done on a per-sending-account basis. ie Each of you clients can have their own subaccount in these providers and only the infringing account will be impacted.