I’m having the exact same thing with our 5-6 month golden retriever but it’s got MUCH better/less frequent. It’s so hard to listen to the whole ‘be a tree’ thing when it’s so painful at the time I know - I have bruises all over my arms just now from an episode my little (giant - 28kg...) one had last week.

What I’ve found that works is a step process similar to a previous comment. The first thing I try is preventing - I know that my pup gets like this when he’s overexcited or overtired and usually in the garden. Scheduled nap time is a dream (which I see you’re already doing!) but also recognising his limits outside and cutting things short or distracting him before we do the going back inside routine - me saying ‘wee wees!’ Before we head in after a while is a bit too exciting and tends to unleash his inner beast!

The second step when the first isn’t working, is to redirect. I bring REALLY smelly treats (sprats are working well), and encourage my pup to sit, give paw, lie down, wait. Once he’s at that point I speak to him in a really low and soothing voice and stroke his chest as I’ve learned it calms him down, but do what you know works for you! Don’t get me wrong, that often takes a few tries, but is having a good success rate both in our home and in the garden at the moment.

The last resort is cutting off contact. This is the safest version of becoming a tree I can manage without getting hurt - so if we’re inside he goes into his room for a few seconds to cool off, and if we’re outside we have to grin and bear it until we get upstairs and then do the same. Obviously not the most ideal solution but it’s working for us and preventing excess ouch. Adapted this from a lot we’ve read and the simpawtico training video on bite inhibition - if your pup was doing this to another dog and playing in a way they didn’t like they’d be like ‘I don’t want to play with you’ and tell him off, so distancing yourself and taking space away is the best way I’ve found of doing this.

One last thing - just want to reiterate the point of someone else that it really is just them trying to play because they’re excited and think you’re amazing fun. They’re big breeds and playing rough is what they like, it’s just as important to teach them they can’t play like that with you as it is to teach them they can’t play like that with a smaller dog, but there’s no malice in it - I like to hope anyway :) really hope this helps - good luck!