My take on it is inspired by the Compose key in Linux. I use dead keys for accents, placed in logical positions. So the AltGr key plus ' and then an 'a' will produce the 'á' for instance.

For frequently used characters, I have 3 special positions (for me, that's the 'æøå' letters; for you, something else like 'áêã' or whatever you need most often). I try not to make much more than those, and keep them in these positions for consistency.

Here's how it looks then (but you can't see the dead key maps on this image), more or less:

If your fingers can't handle the use of an AltGr key, some modification would be necessary. You could swap the dead key mappings with the plain symbols that I've left in there because they're needed for coding and suchlike! So a programmer would be sad if the tilde wasn't available by just pressing the tilde key but you might not care and instead want the dead key, right? Right. Same for the circumflex on shift-6. But the acute might be worse - taking away the apostrophe is too drastical. You might use the slash instead since that's easy to remember as it's a line with the same slant as the acute. That's the kind of thinking I used for my grave accent (the backslash and the grave both point downwards).

One suggestion to make using the AltGr key much easier is to implement a Wide ergonomic modification (search for it on these forums). Then your right hand moves a step outwards so that your thumb is better placed to use the AltGr key than before. It will take just a little effort to get used to, but I like it a lot.