The Short of it:
Can I self-manage my own mass e-mail marketing through the SendGrid "Email API Plan"? Or do I have to set up a "Marketing campaign plan" even if I want to build out my own marketing campaign app.
More details if needed:
I noticed that a marketing campaign plan for 50k emails a month is like $60, while an email API plan is like $20 for 50k emails. If possible, id much rather create my own app and send out mass emails for my newsletter and marketing using the email API because it's cheaper (and also seems like it would result in much more control over personalization and such).
However, the documentation they provide makes it really unclear to me if this is allowed or not. It says you can use the build-in SendGrid marketing campaign tools OR build your own app using the API, but both "Email API plans" and "Marketing campaign plans" have API it seems, so I'm not 100% sure what they mean by this.
Any help is much appreciated.
You can do that, it's certainly possible to send mass emails using the Email API if you want to handle the list side of it yourself. Just note that the maximum number of emails you can send in a single request is 1,000, but if you were sending an email to a list handled by SendGrid, you could make a single API call to handle that.
To address some of your other points, I don't believe there's any difference in the personalisation available to you between the Email API and the Marketing Campaigns plans. The only difference when sending is that Marketing Campaigns use a template always, whereas regular Email API sends can provide the full text/HTML they want to use in the API request. But you can always create the template ahead of time with that HTML for a campaign.
The Marketing Campaigns product has an API that you can use to manage campaigns. That API is not available if you only have an Email API plan.