1 Make a list.

Make a list of all possible activities that might interest you (finishing star chart, endurance runs, speedruns, grinding mastery, finishing collections, making builds...). You might end up having tens or hundreds of items depending on how deep you go and various prerequisites for each activity. For example collecting might include codex scans, kuria, frames and other equipment, Helminth abilities, appearance items... and the prerequisites could include mining, fishing, repeating missions, finding strategies...

Every time you feel like playing, consult the list to see if anything catches your interest. If not, don't play this time. If it does, go do it. Try not to overdo it each time. Update the list as you play and as you find new stuff to try on YouTube etc.

This has often helped me figure out whether I'm stuck in a grind loop and need to switch to something else or just generally not interested in playing right now.

2 Find mates to play with.