Okay, Gmail is probably going to give you the broadest range of options there. It integrates with all of the big names in web automation (IFTTT, Zapire, Integromat), and you also have access to Google Scripting for Gmail.
For web services, check out the three mentioned above and Gmail triggers in those services.
If your always on Mac is pulling in your mails, then you should be able to write some scripting to do some checking. Apple Mail has AppleScript support and rules, and you could then trigger say a curl-based script to do your simple web interactions.
Yes. You are only interested in sending via those three protocols, and not via other options. From your Mac, you might be able to hack something together for messages. For SMS, many of the web services offer something, but this can be dependent on your geographic location, and you may need to find an additional web to SMS service to integrate there. MMS doesn’t seem to be relevant here from what I can tell of what you have described.
Using your Mac would make that option easiest. If you want something online, you could also consider spinning something up on AWS, Linode, Azure, etc. to service your needs there and would probable open up wider automation options for you.
Google (Apps) Scripting is effectively JavaScript, so it sounds like that might be a good fit for you.
You would need to research that based on whatever country you are based in. T-mobile currently operates cellular networks in five countries, and services vary by plan and geography.
There’s been an interesting development around this recently that might be useful to scan over.
You can set up a personal automation trigger based on sender and subject contents, but it now looks like you’ll be able to also process the content which might allow you to cover your “various criteria” requirement that you asserted in your original post.
Shortcuts can certainly send messages via the Messages app and trigger web interactions, but the authentication support is extremely limited in Shortcuts and many people use an intermediate server to handle authentication traffic, so it sounds like you would have to consider that if you took this approach.
These threads might be useful for example:
There was a third party app that was offering some OAuth handling that I was testing at one point, but I don’t think it ever made it out of beta.
You’ll also need to experiment and craft any shortcuts carefully. Some actions cannot run without approval from the user/having an app open , which means having your device unlocked, etc.
You may also want to take a look at PushCut’s automation server feature. Using this you could mitigate some of those challenges.
Shortcuts’ Get Contents of URL action supports GET, POST, PUT, PATCH and DELETE; but not OAuth, etc. Hence the point above about authentication and using an intermediate server to handle the authentication requests on behalf of the shortcut.
Shortcuts can evaluate JavaScript in a Safari session, so it should be able to do some basic stuff again.
Scriptable also has good integration with Shortcuts and is a JavaScript app for i*OS - check out the Scriptable category on this forum:
That would probably make it far easier to do the interactions you are thinking of from Shortcuts. However, please note that Scriptable is not a browser, so the JavaScript object model for browsers does not apply. Do read the documentation.
I think that covers everything you asked about, but do highlight if anything had been missed.