Hi ! Sorry for taking such a long time to answer, we're all pretty tired, we've given our everything to do this game :)
I personnaly did the powerUps "icons" (tospot46 did the cards design and Cahdmus did all the rest of the pixel art I think) so thank you personnaly for this :)
In the new version coming in a few days on the page (along with a patch note because there's a lot of small new stuff) I've added a function that spawns the fishes at their normal sizes and makes them size up to the upgraded version shortly after they spawn. I think it's a great idea !
For the rest of the visuals for the upgrades, I agree it should be more clear how much and what is upgraded by your cards so we'll work on that soon !
I (personnally) think the timing for the big fishes is alright, it might need small tweaks but I think the challenge lies in fishing enough fishes by the end of day so maybe the difficulty ramps up as a downside to an upgrade that's not there yet ? To me, the better you are, the easier it is for you to fish fast but you might still need bigger fishes and/or more strength etc to be able to reach the weight requirement before end of day.
You're right that we might need to be less forgiving the more you progress maybe or something like that, also we might need to tweak the stats on the card to make them more impactful, then bigger fishes would be even harder as the progress bar would be faster and faster. Tell me what you think !
Thank you again for your input and have a good day :)