In my view, “intelligent” pretty much means “able to adapt”. So you are already agreeing that there is intelligence in biology (with what I mean by “intelligence”).
When we talk of “design” we typically think of making a plan, maybe on a drawing board. And when we have completed the design, we prepare the manufacturing process to stamp out copies of that design.
Biological organisms don’t seem to me to fit that. They do not appear to be manufactured.
There’s another way that intelligence is used in ordinary life. We call it “craftmanship”. Somebody intelligently crafts an object for his own use or for that of a friend. There’s more of a personal touch in craftsmanship. And there’s a lot of trial and error testing during the crafting of the object. But it does not require the kind of conscious planning that we expect to be involved with design.
May I suggest that instead of “intelligent design”, you should be considering “intelligent crafting”. And may I further suggest that evolution really is a theory of intelligent crafting. A biological organism is crafted partly by its parents, partly by the population, and largely by itself.
If you allow for “intelligent crafting”, then think it actually is in the mainstream. And “Evo Devo” is an important branch of biology that studies this intelligent crafting (though it doesn’t use that terminology).