I'm joining the party, if I may?!
We had very similar issues with Jack at this age. And for a while afterwards I'm afraid. I've never been quite sure what made them stop, but they did stop suddenly just after he turned one. I think I figured out in the end that we got less when he had more A time but they didn't really go away til one week we got really tough on resettling at night (we got quite strict with WIWO). We obviously had some long nws as a result, Jack slept later in the morning and then cut his day sleep to around 45 minutes / 1 hour. Since then we've only had one nw to settle I think. I'm pretty sure the drop in day sleep was a major factor. At 7 months I think jack was doing around 2 hours, though like I say, we still got some long nws so it was maybe still too much.
In terms of things to try, I agree that it's worth trying a later BT. I've found with Jack that a 13 hour day works best for us. Like you, if we weren't having long nws we had ews, so we've had to keep BT to 7ish. Jack was also on about 4 hours A at this age if I remember correctly. I think we settled on a short AM nap and long pm nap. Although jack prefers longer mornings over long afternoons, I still had to give him one long A somewhere in the day, so we did a short UT morning nap (30 minutes I think), then about 3hrs 15 to a long nap in the afternoon. Though a long nap for him was only 1hr 20-40. He then needed about 4hrs til bed I think. I know at one point at around 8 / 9 months I realised he couldn't sleep past 2.30ish in the afternoon without it affecting nights.
He's always settled straight to sleep for naps and BT as well btw. I think it's just him. And although jack has never done mega naps, lily (also slightly lsn) did. I always had to cap her naps except for just after transitions. I think some LO's are good at self regulating and some are good at napping! And some are good at both if you're lucky, but not mine!