I could have wrote your post, word for word, OP!!! i'm going thruogh.the.exact.same thing but i have a 10 month old baby in the mix too!! so between my 3.5 waking us up 2-3 times and baby 1-2x, i'm exhausted!!
for us, since we moved our oldest into his bigger new bedroom with a big boy bed a year ago, he slept in his bed after our nighttime routine of bath, potty, reading books then lights out. He had a nightlight in his room, door is closed but not locked as he hadn't mastered opening his door yet.
over the summer, he started dropping his naps a day here and there to regularly 3-4 a week, with only 2 naps a week, but 3 weeks ago, once he realized he could open his door....a nightmare for us now!!!! he comes into my room saying he wants to sleep with us, wants to read in bed, to saying i'm scared of the windows so i must sleep with mommy, etc. he's potty trained now but wearing pull ups until we run out of our current supply then go dry. I NEED to figure out a better system, as i can't lock his door especially when he needs his potty.
so for now, we've put a gate in his doorway and leaving his door open. That seems to work for us, as he would just yell for me to open the gate around 6:30, i told him that if he stays in his bed EVERY night for next 2-3 weeks we may remove the gate..to gain our trust back...so im hoping it'll work??!
just writing in to say you have my deepest sympathy!!!!!!