(((Hugs))). We are going through something very similar right now. It's rubbish isn't it? What I'm thinking with Jack is that there's a point where you're capping the nap, but it's not enough to affect nights, so overall they're not getting enough refreshing sleep and OT builds. It's the classic UT/OT loop in fact. We've been round it many many times.
If it helps, the next thing I'm trying with Jack is another 15 minutes off his nap, but probably starting it at the same time as normal so I'm increasing his A before bed. He's EWing, so his A before his nap is already higher. We're trying a 45 minute nap today.... I'll let you know how it goes... So I think that since you've tried long naps, and capping a bit and it's sort of helped but not really overall that it's worth trying a 1hr nap next. That's what my daughter needed at this age. Five minutes over and it'd affect her nights. Although it sounds scary, you've got to just keep trying different things, give them a week or so and then try something else... If a 1hr nap makes things worse, at least you'll know where you are, you can fix it and try something different after that. You'll probably have an idea after 3 or 4 days whether things are improving or not Tbh. With Jack it's usually day / night 4 that changes kick in.
Anyway, I'm happy to trail blaze on this one and let you know how we're getting on? Jack is probably lower sleep needs than your LO, but I can tell you whether capping more is helping us. He'll have 2 nnds at nursery on Monday and Tuesday too, so it'll be interesting to see how they go too after an EW Monday morning (I'm assuming).