Waldo, Welcome to the forum.
The bike you are proposing to use shares exactly the same engine with it's brother the venerable XT600E therefore it is clearly more than capable of pulling you and your luggage around the world.
I would NOT drop to a sub-600cc bike considering your height/weight plus need to haul luggage. A Yamaha TT6/XT6 produces 45 BHP (and most importantly - lots of low down grunty torque) which is more than capable of hauling you and your luggage around Africa. A smaller engine might not pull you out of the deep muddy ruts you will encounter.
The reference above to a TTR600 is causing some confusion on another thread about the TT600 RE! It appears the TTR600 never existed?

If you know otherwise then please join the other thread and enlighten us all.
The only difficulty I forcee is sourcing a TT600 RE in good condition and low milage. Not many of these were sold. They were an upmarket/delux version of the XT600E.
You might be wiser finding a cheaper more popular Yamaha XT600E instead. Plenty still around in good condition and they tick all your boxes above. You can find mint examples for £1,600 or so if you look hard (private buys).
A more modern XT660 will set you back much more (£3,000 plus) and you won't be getting much more for your money in terms of capability / performance. In fact, most here will confirm that the simpler XT600 engine is a far more reliable engine because it's so much more simpler.
I would look at the XT600E first, then the Honda XR650 and finally the Kawasaki KLR650. If you wish to go offroad then look at the lovely little Kawasaki DRZ400 (although it won't be so happy pulling heavy weights around).
It all depends upon how much you have to spend really. The Number 1 bang for bucks bike is without doubt the venerable Yamaha XT600E. It's the most widely used
budget trans-African / RTW bike to be found. And I'm not saying that just because I own two of them myself.
The Yamaha XT600 series of bikes have won many Paris Dakar rally's (when it was held in Africa).
I myself did consider replacing one of my XT's with a more modern XT660 but when I looked at one I realised I would be spending £4,000 for almost no gain!
If you can find a TT600 RE in good condition it will certainly take you around Africa without a problem but the similar (more robust and agricultural) XT600E was made for that job.
It would also be a great pity to destroy a TT600 on a hard-core RTW because they are really quite a rare bike (in the UK at least). I remember people often used the excellent little DR350 for African trips and these things in good condition are hard to find because of this.
If you choose a TT600 or a XT600 there are adaptions to be made - both will need a larger Acerbis tank (it's a safety MUST for Africa) and a sump protector plate needs to be properly fixed. After this the bike will be ready.
I wish you well.