I've been using my 8 Plus since more than 2 years now, battery life is still holding up (%86) but since warranty is ended, i consider switching to a new model. Is it a good idea to switch now or should I replace the battery in future and go on?
I'd say don't upgrade.
There is no direct replacement for the 8 Plus. The 2020 SE is a straight replacement for the regular 8, so unless you don't mind downsizing, it's probably not what you're looking for.
The XR is more of the "upgrade" for Plus users, but right now, imo the XR is a poor value. Not only it costs $200 more than the 2020 SE, it will be supported a year less vs the cheaper 2020 SE.
If you want to upgrade now, the iPhone 11 is the best choice, if it's within your budget.
If you don't mind a smaller phone, the 2020 SE is also an excellent choice.
Only go with the XR if you must upgrade today, don't want a smaller phone, and cannot afford the 11.
Since we are only about 3 months into the release of new iPhones, best bet if to wait for the 12 lineup. At the least, you will be able to get the XR or 11 at a discount. I'd say get your battery replaced to hold you by.