It may be me, or Reddit could be flaking out, but it appears that everyone except you and I have deleted their comments. I also cannot reply to your comment about N8N, so I am commenting again. Weird...

I recommended Pushover in my previous comment. To address your initial question, I'm running Docker containers for Mailrise, which provides an SMTP gateway that embeds Apprise code, and also Apprise standalone. I use Mailrise for services that support only SMTP notifications (e.g. Proxmox). Mailrise translates an email message and, using Apprise code, submits it to Pushover. I use Apprise standalone for receiving notifications from Healthchecks, which monitors cron jobs. Healthchecks supports a number of notification services, but standalone Apprise was the best fit for me.

As I understand, Apprise has its own light-weight HTML API for creating and configuring notification endpoints. It can create a webhook endpoint, but I don't believe it receives messages as webhooks.

Like me, you probably will need multiple services. I tossed up an N8N container out of curiosity. It can accept a webhook and then act on it. It also supports Pushover. So, it looks like a webhook to N8N can be translated to a Pushover notification. Using Mailrise (for SMTP messages) along with N8N (for webhook and other messages) should provide the extensibility you're looking for.

Again, I highly recommend Pushover. Receiving notifications from many different sources, that are organized into "applications" with their own icons, in one mobile app is a tremendous benefit... to me anyway.