Same situation as you in my first pregnancy except my waters went at 31+3 weeks and DS arrived 32+2 after a super-fast natural labour. Definitely no infection present (swabs and urine clear plus have thankfully never suffered from a UTI in my life) and I don't fall into any PPROM risk groups so still don't know to this day what caused it because like you, cervix long and closed when waters went and stayed that way for 6 days after.
I had several bouts of "unexplained" vaginal bleeds after 20 weeks but that was the only indicator of potential prem arrival. A bleed at 27 weeks prompted them to give steroids (which I do think helped him no end).
This time round, I insisted on Consultant and midwife care under NHS at first antenatal appointment and consulted 2 private obstetricians prior to conception (am tight with cash but this is the one thing I wanted a few opinions on).
As you said, consensus really varies and "preventative" treatment for PPROM specifically as opposed to prem birth differs so much country to country. Mainly - in all fairness to the docs - because the reasons are so unclear if you're not in a risk group and don't suffer from infections.
I am currently 33+3 but have had a fraught time of it. I had a large (again - unexplained) bleed at 20 weeks and another at 21 weeks after which I was put on progesterone - cyclogest 400mg and recently doubled to 800mg (the theory being it will help prevent smooth muscle contractions which in turn may prevent "mechanical" rupture of membranes) and twice-monthly cervical scans by Consultant (although she herself acknowledged that this is a measure for preterm birth caused by cervical incompetence rather than PPROM).
I've also read a lot of studies giving mixed results for vitamin E, C and zinc supplements (the theory being they contribute to the production/strengthening of collagen in the amnio sac). Again, I checked for contraindications with Consultant and private docs and all seemed quite content there were none so started taking those at 20 weeks too. (Nb. There was some evidence to suggest too much Vit C early on in pregnancy can actually INCREASE chances of PPROM).
I also gave up work early, rested a lot more and drank a ton more water with this one. I wore a maternity support belt from the outset in the hope that it may help distribute the weight (just in case the amnio sac is weak for whatever reason)
I've also been super vigilant about UTIs and general downstairs hygiene and have avoided baths/swimming pools. I insisted the Consultant swab for bacterial vaginosis too (negative thankfully). I've also submitted urine to the GP every other week between midwife/hospital appointments to make sure it's clear. And have been super stringent with dental care as again, there is a link between gum infections and prem birth - not specifically PPROM though!).
However, despite all this, I have had a load of painful contractions since 30 weeks but they're definitely not Braxtons as they frickin KILL. Still no dilation or thankfully rupture so not sure what's causing them and I have opted not to have any more of the fortnightly cervical scans for fear they'll stimulate more contractions). I'm pretty content my cervix isn't the issue as the scans have always showed long, closed cervix and no funnelling.
I had a positive fFN test 3 weeks ago following suspected preterm labour (it wasn't - the contractions stopped after 8 hours) and so the doctors decided to give steroids and tocolytics just in case. Prior to that, I had 2 negative fFN's which, though not 100% conclusive, did give me some piece of mind.
I also had what I think was my bloody show yesterday so clearly this baby wants to arrive early but is so far holding steady.
So, in conclusion, I actually have NO idea if anything I've done has helped maintain my waters and this pregnancy or whether it is pure fluke that he's still in. However, I feel better knowing I'd done everything in my power even though it felt (feels!) like a lot of effort.
Sorry for super long post but as you can tell, this is an issue very close to my own heart!