Hi, I can totally understand what you're going through and know what you mean about knowing your LO can be better off with more sleep. I was exactly the same.
My ds struggled with 40 min naps from when he was 5 weeks until he was about 4 months. After trawling through the BW site, a NZ baby site and asking everyone, I discovered what was going on and how to fix it for my boy.
Like you have thought, I was overstimulating when he was awake, from under the play gym, to the jolly jumper, a bit of baby mozart etc. Never any 'quiet time' just on the floor by himself or with me. I was always giving him something to do. After 1h 15mins awake he'd be yawning and rubbing eyes to go to bed. So off he'd go, cry for about 5-10 mins then fall asleep for 40 mins exactly. He'd wake up crying mostly but would not resettle even though I could see he was tired. So, up he would be for 1h 15 mins then back to bed. This would go on all day.
Someone pointed out that due to his shortish awake time when he got to the 40 min mark in his sleep cycle which is a light sleep time, he'd wake up through the transition and think "hey, I'll wake up now, because I'm not tired enough to go back to sleep, due to my short awake time". So, I decided to keep him up as long as possible without him having a break down, but do it with really low stimulation. Infact, the whole time we did nothing 'stimulating' for him. It was just stories or walking around the house/garden. He stayed awake for 1h 45mins. Was exhausted when he went down so much so he didn't even cry, just closed his eyes and went to sleep.
He slept for over an hour! I didn't even hear him at 40 mins. I was so surprised, happy and relieved. So for the next few awake times I did really low key and kept him up 1h 30m to 1h 45 mins, depending how he was going.
He can still only do 1h 45m while at home but lasts longer if we're out.
His naps are usually 1 hour for the first, up to 2hours for middle of the day then 1 hour for the early afternoon. I also usually go in to see him smiling and laughing, not crying. Sometimes there's a few tears but only until I pick him up. Not like before.
After 2.30pm once he's woken it's a rare occasion that he will have another nap in his cot. He'll sleep in his buggy while walking for about 30 mins, but that's the only way to get him to sleep. So some days he can be awake from about 2.30pm until bed at 6.30 -7pm. These afternoon can be a little tricky sometimes, but hey, his other naps make it all worthwhile!!!
This worked for me, can't guarantee it will work for you or others, but I hope it does! I battled these naps for so long, I knew he'd be happier if he had more sleep and he is. All the best.
PS I hope you haven't fallen asleep from reading this rather long novel! :lol: