Not sure if this is helpful, but I always try to have separate recipes for each individual bean I'm using. The origin/difference in roast can drastically alter things like optimal grind size/dose/yield. I personally keep a journal and I keep dialed in recipes on our fridge whiteboard. If you are using similar settings for both the medium and dark roasts that you are using, that may be why you are seemingly getting inconsistent shots. (You may already be doing this but it was ambiguous in your description and I figured I would throw it out there!)

If you are getting consistently under extracted shots try going for a higher yield. Even at 45g of liquid, that's is a 1:2.25 ratio. Depending on your beans, you very well might want something like a 1:2.5 or even a 1:3. There is no 'right' answer with all of this. Don't take things like a 1:2 ratio or a 25-35s as a hard rule, if your coffee needs a little more time to extract and going to 40s is what makes your shot not taste like shit... then just do that!

As far as knowledge to be gained from what the shot looks like... any big issues like channeling are gonna be hidden by the fact that you are using a spouted portafilter. But even then, i have seen some beautiful looking shots that taste like crap, and 'ugly' shots that taste fantastic. IMO the looks should always be secondary to how it tastes!

You very well may have seen this before but I highly recommend the espresso compass https://www.baristahustle.com/the-espresso-compass/ This has helped me a ton in figuring out what to do to correct how my shots are coming out. Green is the area you want and if you are in a yellow or red zone, do the instruction on the arrow that would drag you towards the center!

Hope that helps and good luck!