Feedback [WIP] Bubble Calculation

Hello developers! I recently started developing mobile games and have been working on math game that is similar to the hit game Flow. I wanted to make a game that focuses on mechanics rather then visual because I am working on this solo, and am not the best artist.

Description:
You will be given an equation to solve. You solve this equation by matching together number blocks in a grid like pattern. The equations will progressively become more challenging. The grid size along with the range of numbers available will also increase making for a wide range of difficulty. There can also be multiple equations, where you have to find the numbers that will make both equations true.

Game Modes:
There are three different ways to play the game. First, is simply play each level as is. This will get you familiar with the mechanics of the game and get your mind prepared on how to solve them. The other two require you to complete all the levels of the desired game mode you wanted to play. Free Play will just select random levels for an endless kind of play style, I found myself playing this the most as it is a great way to mindlessly pass the time. The last game mode is Time Attack. In Time Attack you play though each level in the given difficulty and you have to see how fast you can complete them all.

Notes:
In the top left of the menu screen there are buttons to reset to the beginning, along with a button to unlock everything if you didn't want to spend your time unlocking every level. This should be a given but I will say it anyways. This game is WIP and as such there are bugs and not everything is fully function.

Some things I would like to ask:
There are a lot, but I will try my best to not flood you guys. What do you think about the concept? I have spent a lot of time prototyping and changing this mechanic to try and make it more entertaining. What about the visuals? I wanted this game to be playable by all ages so I opted for a fun and simple style, BUBBLES. What about the difficulties, do they become challenging too quick? My biggest fear for this game is that there are not enough levels. I wanted each level to feel different but I am limited in what kind of equations I can use. I didn't want to repeat the same equation just on bigger maps with bigger numbers to the point of repetitiveness. Along with that I fear how hard the game can actually become. The hard difficulty would require multiple minutes just to solve one level to the point of frustration. Lastly, I developed the game in a way where I could implement user custom equations. What are your thoughts on that? Should I add a social aspect where you could share your equations with friends?

You can play the game here:
https://razorrules.itch.io/bubble-calculater