If you think this "very hard".
Try this version instead:
https://meteorviix.itch.io/mask-idle-venture-fix-balance
Quick fixes and changes in this version:
1.Increased starting HP from 50 → 80
2.Fixed a bug with selected card values (the issue was actually incorrect logic in the code)
2.1.Reworked the assumptions/calculations based on the reference image
3.Added a new feature: Card Stacks
3.1.When you collect 5 stacks, you gain +1 extra card
4.Adjusted game balance to make the 3000–6000 distance range slightly harder than 0–3000
5.Fixed enemy scaling at higher distances
5.1.Enemy size growth reduced from 0.5 → 0.2
5.2.(enemies grow less as distance increases)
6.Reworked the EXP requirement formula
6.1.EXP required increases by 1.2× per level
6.2.Every 10 levels, required EXP is reduced by 40%
7.Add Skip button (X2 / X4 / X8)
Rebalance Part 2
1.Reworked Critical Chance Card values to 5 / 10 / 20 / 30 (previously 1 / 2 / 3.5 / 5)
2.Reworked Critical Damage Card values to 1 / 2 / 4 / 8 (previously 0.2 / 0.4 / 0.7 / 1)
3.Increased player default Critical Damage to x2.0 (previously x1.5)
4.Reworked Max HP Card values to 20 / 40 / 70 / 120 (previously 10 / 20 / 35 / 50)
5.Reworked HP Regen Card values to 2 / 4 / 8 / 15 (previously 1 / 2 / 3.5 / 5)
6.Reworked Movement Speed (MSPD) Card values to 20 / 40 / 75 / 125 (previously 5 / 10 / 18 / 25)
7.Reworked ATK Card values to 1 / 3 / 6 / 10 (previously 1 / 1.5 / 2.2 / 3)
8.Reworked Attack Speed (ASPD) Card values to 0.2 / 0.4 / 0.7 / 1.0 (previously 0.1 / 0.2 / 0.35 / 0.5)
9.Every 3000 distance, the character automatically gains +10 MSPD
10.Reduced Bonus Stack Card requirement from 5 → 4