I am in a similar situation as you. Have a Mid 2012 MBA. Works fantastic. Also Catalina beta seems to have sped the system performance immensely. Always plugged in because of the battery.
Recently replaced my 2012 MBA battery, the batteries are quite cheap and the replacement is surprisingly quick and easy on this model. About a five minute job, you can buy the battery with the needed tools here: https://www.ifixit.com/Store/Mac/MacBook-Air-13-Inch-Late-20...