Packing for the TMB, last 2 weeks of August - need help!

The TMB is a well formed trail that is easy to follow and walk. There is a good deal of up and down but the trail itself should present you with no problems. This also applies to the variants. In late August the trail will be busy and you will rarely be alone. You definitely do not need a compass and I doubt if DEET will be necessary.

For the TMB type of trail I use my 45l climbing day pack. You could use a smaller pack with no problem. My pack would certainly not weigh more than 10kg/22lbs for such a walk and would probably be around 6kg/13lbs to 7kg/15lbs if I did not take my DSLR. You do not need a sleeping bag, just a liner for use with the hut coverings. For clothing the rule of threes works well for me; i.e. three sets of underwear, three pairs of socks, etc. "Suitable weight" is a personal/ideological choice, just try to make it as light as possible/feasible for you.

GoreTex is breathable to some degree but late August will probably be warm with the odd thunderstorm. Consequently you do not need a heavy, all-weather jacket. A lightweight jacket should be fine. As you say, a fleece underneath will provide the warmth, particularly in the evenings. Wooly hats are always good.

Footwear is often a vexed issue on TT. My view is that you do not need a pair of leather boots for the TMB. I would use a pair of light, GT-lined, boots. Others use walking shoes and I have seen running shoes and even sandals used successfully, although sandals seem a little too minimalistic to me. Use what you are most comfortable with but remember that insufficient padding will lead to sore feet. IMO the only reason for using leather walking boots would be if you will need them for some walk in the future and do not want to buy two pairs.