Sorry Dennis, I was misled by your sig. I assumed that we were talking about the 100-400 II or the 400/4.6. The 300/4 is a wonderful lens, but once you add a teleconverter, it naturally drops back to merely good, possibly very good. The Tamron 100-400 is good for a cheapie but not in the same class as the Canon 100-400 II, the 400/5.6, or indeed the 300/4 bare lens.

Looking at those three pictures, they appear to be about as good as you can reasonably expect from the lenses you used. The Tamron 100-400 is a little surprising to me, I'd not have expected it to be clearly superior to a 300/4 and 1.4 converter. (But I'm assuming (a) that these shots are representative of the two lenses usual output (not just lucky or unlucky flukes) and (b) that the teleconverter is a good one, well matched to the 300/4. I.e., the Canon one, which is designed specifically to go with the 300/2.8, 300/4, 400/2.8, 500/4, and 600/4. Not so sure about the other teleconverter brands without testing them one by one.)

So: can you improve on these shots? (Without getting any closer, I mean.)

Yes: you'll get a small but consistent and significant improvement by going to one of the very best 400mm lenses (Canon 100-400 II, 400/5.6 except that it lacks IS which is a deal breaker for most people - rightly so in my view), or 400/4 DO. Better again a 500/4 or a 600/4. All of the last three mentioned will take a teleconverter happily. But don't expect miracles - with the extremely high pixel density of the 90D, teleconverters really won't help a lot. As a general rule, I put a 1.4 converter on my 600/4 OR swap the 5D IV for a (higher density) 7D II, not both. And although I own a 2X converter, I hardly ever use it, not even with the 1D IV or the 5D IV.

Hope this helps - Tony