One note about that mid roundhouse kick is that your body positioning is going to be a bit weird because as you jab, your opponent steps to his left, he'll be more towards your right. So if you throw just a straight roundhouse, it's not gonna have much impact because it won't have time to develop speed and strength from your hips. So there's two ways that you can make that kick effective.
(I'm assuming you're a right handed fighter in a left foot forward stance).
1) After your jab is extended, on its way back, take a small step forward with your lead foot (left foot). That will set up your momentum and give you extra "oomph" in your roundhouse impact. It's like the roundhouse that Bas Rutten advocates where you step out with your foot towards his outside. If you need more details, you can try doing a search on the forums for Bas Rutten's roundhouse kick. It was an interesting debate a while back.
2) Another way you can set up more power in that roundhouse kick is to take a quick, small shuffle step to your rear left as you jab. This is a bit difficult to explain, but I'll try. As you jab, your rear foot (right foot) moves behind and to the left of your lead foot (left foot). Since you did TKD, think of it as the step through sidekick. You know the one where your rear foot steps behind and crosses your lead foot and then you do a side kick with your lead foot? If not, watch the old Bruce Lee movies where he does his "power side kick." He learned that from TKD. Anyway, after your rear foot crosses your lead, the lead foot (left foot) takes a small step forward while pivoting towards your opponent. This will set you up better to throw that hard roundhouse. Because if you're sparring Muay Thai, tapping kicks ain't gonna do nothing. You have to throw it hard.
So I recommend you try these two leads into the roundhouse kick (if you choose to roundhouse him). But the second one is definitely hard to time and use. You need to pretty much step, pivot and kick perfectly in order for it to be effective.
Also, you mentioned that sometimes the guy hooks with his left. For that I would say try the same thing, but instead of kicking, throw a knee into his exposed rib since the distance will be closer if he's stepping in to throw a hook. The only thing you have to remember is to keep that guard up or you're gonna eat his fist while you're kneeing him. Try to block the hook with your right hand; grab his neck in an extended clinch with the left; and pull him towards you as you knee his exposed ribs.