Before modifying anything, such as alloy cam covers, insure the basics are correct.
Mainly that ALL the breathers pass air/fumes in both directions via the hoses at the back. At risk of repeating, if they don't flow air, work your way back to the offending blocked holes either in the plastic Eco tech box, which can easily be removed and a paper clip forced through the holes and cleared. But don't make the holes bigger in any way or the engine will use too much oil.
If you can't clear the alloy breather box with small implements then you'll have no choice but to remove it and investigate further. The oil film and gunk on the inside of that box can dry to an exceptionally hard and brittle film that can cover the small hole on the inside. It just films over and sets. I can't explain about the large bore hole as I never had issues with it but I have seen pics of it fully gunked up with crap.
If alloy covers are fitted and the problem isn't rectified the pressure will simply burst out somewhere else. Next in line is usually the front or rear crank seals. These are awkward, time consuming and expensive to replace. Beyond the scope of most diy mechanics. Although some here have done so.