Bought an iPhone 13 Mini brand new about 3.5 years ago when it came out.  I remember the battery was really good.  Last year I had to get a screen replacement on it due to green lines on it and had the screen replacement done at the Apple store.  I recalled leaving it with them for a bit and coming back later on and picking it up.  I had considered getting a battery replacement as well since I had my phone for 2.5 years.  I recall the battery health was around 86%.  I read you should get a new battery only when the battery is below 80%.  Is that true or false?  Do people here get it when above that?
Now it's been like 9 months later and my battery health is 84%.  Is there a reason it only dropped 2% during this entire time?  Now the thing is I recently used my iphone 13 mini a lot while outside and used it in low battery mode while using my data.  So this could be watching YouTube videos or using Twitter for a few hours straight.  I recalled after doing that for maybe 3 hours or so, the battery went to about 30%. Is that normal when using data?  I know data uses a lot of battery compared to wifi and obviously compared to wifi off.  Now I used it sparingly a bit for less than an hour total and once the battery was around 20% or so, battery drained real fast.  Then once it came to around 11%, the phone turned off.  I would power it on and it does but can't click on anything that opens and it would turn off and it would show 8% and then 5% etc.  Back when I had my iphone new or say a year into it, I recalled it was only when the battery was like 3% or so when the phone would shut off.  It didn't have these issues at like 11%.
So based on this information, should I get a new battery replacement?  I already spent money last year replacing the screen.  I didn't want to buy a new iphone because I did not want a big size iphone.  I prefer the iPhone 13 mini size.  I don't recall how much it was exactly but it was over $250 or so.  Does that sound right?  I googled the price now and it shows Apple charges $229 so including tax, it comes to around $250 or so right or more less?  So I googled and the price for battery replacement is $89 so including tax is about $100 right?  So should I do that now or wait till it's below 80% battery?  Now if I don't use my phone much while on data and just use it on wifi, the battery last pretty good.
The thing is I want to be able to use my phone's data a lot and use it for hotspot for hours.  So with my battery now, it's not that good right? What I did recall was when my phone was new or close to new and battery health was 100% or high 90 percent, when I used it as hotspot and tether internet to my laptop, it would drain about 13% per hour.  So essentially I could use it for 7 hours as a hotspot or so which is very good.  Now it's much less but haven't tested it.
My phone is 3.5 years old almost.  I should be getting a battery replacement for it?