Self soothing

My baby is 6 months old! I have always nursed to sleep. It doesn’t bug me at all. Lately I have noticed that baby nurses but sometimes doesn’t want to drink, just comfort nurse to sleep, also fine, but baby send little frustrated that there’s milk coming out instead of just being able to suck... so.. A little bit ago we were doing our routine & baby popped off and few seconds later started sucking thumb. Now I’m wondering if 1...learning to self soothe? 2.. coincidence or start of becoming a thumb sucker (I took thumb out and nursed to sleep..)

If my baby is starting to learn to self soothe, what ways can I encourage instead of sucking on something. Baby has never taken a soother. What did or do you moms do?