Sounds like you are having fun with it just like it is so why ruin it with upgrades that will do little to improve it. You have other good bikes so why try to make this one like those? I used to have an old beater single speed cruiser I got for 5 bucks and kept it on the porch for years as a grocery getter. It had a giant basket on the bars. All I did to it was keep all existing parts in top tune. Repacked bearings, rebuilt coaster brake, trued wheels, greased crank, etc. It was real fun just keeping it original but in better than new tune. Upgrading parts would have ruined the fun. After the initial 5 dollar investment I put maybe another 5 in it over 3 years until it was finally stolen.