Back when we had The Talk, I was around the same age (10-11) and, same, mum was the only one around to do it. Ske kinda’ tried to pull a workaround and not directly talk about it, instead she asked me what I didn’t know or didn’t understand about the whole deal.
By that time, biology classes had caught me up to the physical side (I knew what puberty implied, the hormonal changes, the growth spurt, the extra hair, the new urges, the new smells, etc.) and, as you’ve mentioned as well, mum and I had already discussed some of the details in random conversation (eg. she taught me to occasionally shave my armpit hair, because it tends to hold the smell of sweat over time, to select face and head washes which worked for my specific skin type - used to be as dry as a cracker, then suddenly I started oozing grease from my pores, stuff like that) and helped me manage my acne with daily face washes. But the meat (pun not intended) of it I’ve basically learnt from friends, literature and porn. However, The Talk rendered no new information. I found that her dumping the responsibility of asking questions on me kinda’ cancelled it out from the start - I didn’t even know what to ask, to be honest.
But I keep thinking back to biology classes and how easy to digest they were due to how clinically the information was presented. If I were to have The Talk with my son (I’m a dude and celibate, btw), that’s how I’d broach the subject - start from the basics, the source (the hormonal changes), then slowly branch out to the effects going from small to large (you’ll grow more hair and start being stinkier -> here’s how you don’t leave someone pregnant). And I’d always keep it open for questions, approaching any which may arise as scientifically as possible. I’d probably make a list of all elements to be covered and try to go through it. And I wouldn’t be afraid of not having an answer, it can always turn into a research session for both of us! Heck, working through the research together may be even more effective, it’d be clear to see that this is a well-known and well-documented change in a human’s life.
Thing is, this’ll most likely start out being awkward for both of you, because it’s awkward for pretty much everyone. But I genuinely think presenting these things ‘scientifically,’ objectively, goes a long way in underlining the fact that absolutely everyone goes through them, nothing’s shameful about it, gonna be weird for a while, but it’s also manageable if they have the info at their disposal.