Yes, what I'm getting at is a way to create a notifications/inbox client that pulls from all the above stated API's but does not attempt to push any content to them, instead switching the user over to their preferred webclient when content interaction begins.

Kind of like how an email client shows a title/headline, sender, and Metadata for each post but then expands a rich text / markup environment when you open the email. Or how some minimal RSS readers just pull headlines and a quick summary of each post but push you over to the original website when you click through to read the full post.

I just want one place to see who has posted on what, and then use the front ends specialized for each piece of content to actually interact.

I suppose there could be a side development to this, which would be the ability to associate accounts across multiple services and then filter only for them, for example if you know someone who posts on lemmy, Mastodon, peertube, Pixelfed, and loops, you could view all their posts of the day in one place. Or a team you are a part of. Anyway, once you can pull multiple services into one inbox the data sort potential is an obvious added benefit.