Looks better now the grass is more blended :)

I'm wondering if it would be better to have wider edge fade inner effects on a map of this scale - to blend the biomes into each other.

Looking good so far :)

It depends on 2 things. How large you intend to use this map, and whether it's practical to scale the river width up when there are so many of them.

For instance, if you want to have it as a large export that is 10,000 pixels on the long side you can get away with more rivers of a slightly wider line, but if you only want it to be 5,000 on the long side when exported, it might be less crowded with rivers if you delete the tributaries and just have the main rivers fractionally larger.

Best thing to do is to export it at the size you intend to use it right now and look at the rivers - see if they are a) visible, and b) cluttering up the map too much, or just about right.

This is also a good time to make sure they all reach either the coast or a very large lake/inland sea.

The Herwin Wielink colour palette on your map isn't really helping you, with only a single row of relatively dull blues to use, so here it is again with a more balanced colour palette attached to it - to give you more choice with your labels (and only if you want it with this modification).