So I want to start out with I hope you don't take all of this as harsh criticism. If I hated or disliked the game I wouldn't take the time/effort to write all this out as it's a lot of work to take the time to do so imo, I would just move on. But here I am, so I want to see this concept potentially go somewhere.
Atm the game is extremely barebones. You're already aware of that however which is good. You have a good concept here (though I'm kind of disappointed it's just the same style of game that's been popping around a lot lately, I hope you try to branch away from it somewhat and become more unique!) but so far the only mechanics are grow/multiply mushrooms & collect a currency. I would recommend looking into implementing more mechanics to keep the game interesting as eventually the satisfaction of watching the mushrooms multiply will wear off and the game will lose any excitement it had.
Some mechanics you should look into are especially active ones. Something other games of this style have over you are they are usually very active in gameplay requiring player input throughout the stage but once you get enough harvesters it becomes too tedius to keep clicking mushrooms yourself and you just take the loss in income. Some examples on how you can make it active are abilities (I.E. exploding click will cause a big explosion collecting mushrooms in a large radius, an AoE ability that you can place somewhere to collect mushrooms for x amount of time in that area, an ability that makes mushrooms worth x times more on click making it more worth looking for the rarer mushrooms like gold mushrooms) or upgrades like collecting currency in the middle of the round increases it worth, clicking the screen speeds up harvesters etc.
Atm the game gets really op with how fast mushrooms ramp up in speed due to multiplying. So I think you should also introduce mushroom evolution. New mushrooms from collecting a mushroom have a chance to evolve into a new type of mushroom that takes longer to grow, however ofc it's also worth more. This will help prevent the issue of there being too many mushrooms later on, needing too many collectors to keep up while also allowing the implementation of more round time upgrades & mushroom grow speed upgrades and even unique upgrades like red mushrooms are worth +x more and white mushrooms are worth +x more. In regards to collectors, their AI is really strange from an efficiency point of view. They can go from one end of the screen to the other when there are multiple grown mushrooms around it, not entirely sure how it works (maybe it targets the oldest grown mushroom) but it would be cool to see upgrades implemented to help curb this, i.e. an upgrade that gives collectors a chance to harvest mushrooms they pass through on the way to their target!
Some cool things I'd like to see if you're interested as well are special mushrooms, like a blue mushroom that has a rare chance to spawn but freezes the round timer when collected. Little creatures that might try to steal your mushrooms if you don't pay attention & click to shoo them away (maybe upgrades to get something like the collector which will help you shoo them away). And different types of collectors, like a more powerful version that might spit out more currency when collecting mushrooms but has a charge & you need to recharge (i.e. clicking on it) or a collector that will collect currency for you with a multiplier but moves around super slow (it just moves around randomly with a "Vacuum" radius around it)
Finally just some minor issues with the game, the UI is a bit of an oof. Mushrooms take priority over *everything*, everything gets put in the background behind mushrooms, your currency at the bottom left, the stage timer etc. This is just frankly kind of weird. Also little mushrooms can grow "on top of" grown mushrooms and they make the already small hitbox of a grown hitbox even harder to click. When you full-screen the game, the locations of mushrooms seems to be inconsistent. Mushrooms that were previously at the near tip-top of the screen are now completely off-screen where only a collector can grab it. Also feel like currency shouldn't be on top of upgrades but below them, so you can take a look at your upgrades properly while collecting the currency after a round end. Finally, not really an "issue" but it would be nice if branch upgrades were visible as like a ? block with x/x on it so you know that upgrading will unlock another upgrade. Wasn't sure if upgrades like gold mushrooms will have more upgrades or not i.e. when fully upgraded or just even partially upgraded. Turns out mushroom multiplication did have another upgrade to unlock when fully maxed, so would have been neat to have known with a ? x/10 block to the left of it!
Thanks for the fun 15-30 minutes it was really satisfying watching the mushrooms multiplying and I hope you really do something awesome with the game, you don't have to necessarily follow/implement the suggestions I have. They're mostly there to brainstorm ideas for your game and what's most important is your own vision for the game! I don't want to interfere with that c: Good luck with the game and I'll try to check back here and there for updates :D
Hey man! This is sooooo helpful, I really appreciate you taking the time to leave these comments. A lot of these these things, and in particular the fact that it loses it's active nature too quickly were already on my radar, but these suggestions are SUPER helpful. I really like the little creature idea. I'm definitely trying to make it more active and I really appreciate this! Thank you so much and I'm glad you enjoyed it even if just for 15 minutes!