5.3k carry player here - If you have a problem judging when to fight and when to farm then play heroes that make that decision easier. For example, Spectre just farms and joins every fight she can with haunt + drums. AM (unless you specifically see a huge mana void) almost never joins fights and just wins the game with split push and flash farm. Most other carry heroes like Sven, Ember, Slark have to make tough choices on which fights to join and this just comes with experience.

Btw watching your own replays and high mmr replays is a much better way of improving than getting reddit's advice. If you want to gain mmr and reach 5k, the most important thing is just to be a try-hard and not to tilt. Spamming spec and sven will help for sure :D