I had Redmi Note 8 Pro in my family. After 3 years, the battery started to swell. I had to get it changed. Its then when I also started using Accu battery. Check if your phone's battery isn't swollen, and even if it isn't you can get your battery replaced. It might be EOL.
78% health after 3 years is good & health degradation is normal.
No matter how much you try to perfect your charging cycle, battery power tends to degrade overtime. 78% Health is honestly a good number after 3 years of usage so you have nothing to worry about.