I don't think he lacks power, I think he's actually pretty heavy handed. His main issue is speed as you say. He's slow and so his opponents see his shots coming, that combined with his pace sees his opponents shelled up behind their guard for long periods of the fight and so able to ride out his attacks. But when he does on the rare occasions he does land on an opponent who isn't in a defensive shell his power looks pretty devastating. Power isn't just about how much force you generate, it's the ability to land it suddenly when an opponent is exposed and his lack of speed makes it hard for him to do that. If he was truly lacking in power his opponents wouldn't shell up like they do once they taste his power. They'd be far more willing to stand and trade rather becoming defensive and looking to survive. Hrgovic right now is the one to watch but I feel he has plenty of flaws too. He's got a more aesthetically pleasing style than Joyce and being younger has greater potential to get better but right now I think Joyce would still beat him like he did in the WSB.
Joyce is not a big puncher for a HW. He's not a say Byrd or Zuri Lawrence per se but he's not anywhere near the hardest hitters of past or present. Probably a 5-6 out of 10. Has a sort of clubbing accumulation to his shots but is not a massive puncher. I've watched every single one of Joyce's fights since he debuted and used to be somewhat of a believer myself but with each passing fight it's gotten harder and hard to ignore the flaws. Hrgovic was only 20 then and many thought he did enough to win. I'd pick him to beat Joyce outright and like you suggested think he has way more upside. Most upcoming heavyweights have flaws. Some are just more profound than others. Joyce at nearly 35 is as good as he's going to get.
I wouldn't say he's one of the top punchers that's for sure, but he has enough power at heavyweight to get anyone respect in my opinion. Personally I had Joyce beating Hrgovic. It was a competitive fight, Hrgovic showed Joyce's liability to be caught with right hands as some have mentioned here is a flaw in Joyce. But I think as the fight wore on we saw Hrgovic struggle to keep up with the pace Joyce was setting. He was landing right hands throughout but as the fight wore on he was taking more and more in reply from Joyce, which is why I think even now Joyce would beat Hrgovic as it would suit Joyce even more in a longer format. Hrgovic for all this strengths still backs up in straight lines and still doesn't move his head enough and until he corrects these flaws I think someone like Joyce who would be able to keep pressing and keep coming would grind him down. But I agree Joyce isn't going to get any better at 35 which is a shame as I think with some tweaks to his style he'd be a bigger threat to the top guys. Right now I think he a tough fight for most but wouldn't beat the top guys.