What do you mean by "without managing them directly"? Are you a manager or aren't you? If you're a scrum product owner, you are (or at least you set priorities)

What do you mean by "without managing them directly"? Are you a manager or aren't you? If you're a scrum product owner, you are (or at least you set priorities). You speak pf "resistance to change", from juniors? You are the change. You get to set the agenda, not them. Unless you don't, in which case you can't fix anything. But legitimacy comes not just from authority, but also from rigor. Anything you truly dictate needs to be 100% based in evidence and fact. This means letting go of implied a-prioris such as "PHP is bad" and "we must use a framework". The only real constraint is to keep the gravy train rolling. So what exactly is your role, the thing you were hired for? If it's to manage, manage. If it's anything else, the best you can do is lead by example. But one way or another, you'll have to let go of some things.