You seem to reflect what I've noticed around here too - there's this vague awareness that the need to allow independence is looming because of secondary, but an absolute fear of how to do it.
Personally, I think you're doing your child a disservice if you'r not proactively getting on top of this now. I also think that while they're still quite young and a bit more willing to listen to you is the time to embed rules and processes so that it becomes normal for them.
So, letting them out. Start with walking to/from school. If you have concerns re road crossing and you haven't already done this, start by letting them "direct" the school run and make all the decisions for you when walking. Then they can go a bit ahead. Then they go alone.
Phones - absolutely I can look at a phone for a child that age at any time I like, for any reason. I told them that I am not particularly interested in the details of their phone conversations r whatever, but I can and will do set checks and they don't get a say. And I did perform those spot checks. (I don't, as a rule, check DS' phone now that he's in year 8, but in theory, the rule is still in place should there be a reason).
Re going out and about. Again, start small - stopping at the park, with some friends, on their way home from school. One rule I was militant about (in Year 5/6 when this was starting) was that he had to tell me if he wanted to do this, who he was with and be home on time. There was a zero tolerance approach to not meeting these basic standards - so home late? No independence for the next week.
As he's got older, these rules have evolved somewhat - he still has to be home on time and he still has to update me on his broader whereabouts, but there's more flex on, for example, when he gets home from school - he doesn't have to be home by exactly 15 minutes post school as I accept that sometimes they're all just lurking outside, hanging out etc. But if he's later than 4pm (school finishes 3:15), I expect a call. Ditto, he if goes out to meet friends to play football and they then decide to go back to someone's house, he calls me to let me know but doesn't have to ask permission or check with me first. If he's heading out for a whole day, he MUST have a fully charged phone with him (I'm less strict, obviously, if he's just heading to a mate's house to hang out).
Ditto public transport. He was desperate to start using the bus. We did a few practice runs. Then he was allowed to take the bus from our house to local town centre (only 15 minutes) but with a friend. He had to call me on arrival. He and his friend DID manage to get lost on their return but he called me, we talked through it and they got on the next bus. Last year he started taking the train one or two stops up or down to meet friends (his school has a large catchment). We had detailed conversations about how this works int he beginning. He was, of course, super casual. He also, of course, landed up on the wrong train. But I expected it so had made it clear that he could ring any time... so he did, we figured it out, and back he got on the right train.