I'm in the same boat as you, except I have experience with it. I'm 6'5" and 182lbs. You want to use your reach to your advantage, keep it outside. Like others are saying, if the fighter gets close, lock them in with a clinch, and throw some nasty elbows or knees. A really effective clinch for us tall guys, is to tighten as much as you can, and yank them side to side, and spin them around, it'll make them lose their footing and leave them open for some powerful knees. With kicks, mainly use snap kicks (where you throw your hip into it like a hip kick, but instead of completely extending your leg, you "snap" at the person with the kick, kind of like a whip I guess, then pull it back really quick into your stance, it damages and doesn't leave you open). As for punches, the jab does work really well, aim at the center of the chin for a knock out jab. A tall guy can use the inside and the outside equally as well if he knows how to. Gets to close, get him with a push kick, hip kick from the side, quick, tight hooking elbow, or a quick, tight, downwards hook. It's all about measuring the distance properly, and knowing how to strike at any moments notice.

Well considering Muay Thai is 80% infighting, I would say go with the Muay Thai clinch and stand tall while trying to knee the head or even body/ribs. If we are talking about you vs the average height Thai, then you can probably down elbow on the head too. You may get stabbed after the fight for that though lol.

Being taller gives you better range but if they circle away from your kick then it will leave you open all day to a nasty leg kick. Leg kick with all your weight on the leg getting kicked is the worst kind.

I'd actually emphasize trying to keep it at a distance. Use push kicks to back the guy off. If you're as tall and flexible as you say, then I'd also work on using head kicks. When he tries to close in on you, you can use knees or elbows, even without the clinch. Don't get into punching exchanges; when the guy gets close, then yes, tie up, and punish him with knees. With punches, focus on the jab especially, but also the cross. Try to keep your distance and let the guy wear himself out by closing the distance, and every time he moves into range, make him pay by sticking a jab in his face or, if he closes quickly, have him run into an up-elbow or a long knee.

We don't know that you have to have an official Thai Mein Instructor.

Obviously everyone has their own style but hypothetically speaking, if someone is 145lbs, 5'11" and flexible what would be an effective style? Would it be best to keep distance with teeps or to get in close and clinch knees to the head or anything else? Thanks.