Its possible that IOS 14 is better at reading the battery's health more accurately. My own 8 Plus, which is essentially the same as your X, currently shows 87% on IOS 13. I plan to change the battery, but using my 12 Mini as my daily driver now. You should really just change the battery for £/$50 , like acorntoy said, their is no 5.8" size iphone anymore, so a fresh battery will give it a new lease of life.
The screen size of the iPhone X. It has been discontinued.
if you want to stick with that size but still upgrade you could buy a refurbished XS or 11 pro directly from Apple ( it’s basically a new phone I’ve had a -lot- of experience). The X will feel great with a new battery though, as it’s cheap I’d recommend changing the battery, and if you don’t feel like it’s improved enough sell the phone. If you make a point that it has a new battery when selling you will recover most of the cost back. Even non techies check iPhone batteries when buying second hand these days, A phone with a new battery will command top price at whatever condition it’s in.