Regarding battery cycle of Iphone

80% after 500 cycles is the design standard so you are probably doing slightly better.

When health gets to 79% as displayed in settings , Battery, Battery health have Appl replace the battery

That sounds all very normal. Using third party battery apps may be misleading but losing 1% capacity (Health) per 4 to 8 weeks is normal.

If you drop below 80% inside the warranty or the AppleCare+ period it is usually replaced free in many countries.