I had a heap of tests done at Derby University mostly related to my strongman training, but I recommend a Dexi Scan, they call them nuclear scales, they use x-ray tech to scan every gram of your body, they tell you precisely down to the gram how much fat, muscle, bone you have, even how heavy your organs are. They can isolate muscle wastage, so if one leg is weaker they can weigh the muscle in the leg and see if it needs attention.
If you upgrade they tailor an exercise program to correct imbalances and posture etc. Little things like using 1kg extra weight on the weak side makes the difference.
The basic you get a 2 scan deal for like £80, and you can do them a month apart or 6 months apart, to see the difference or weight loss, muscle gain, visceral fat loss etc.
I also use Medichecks for bloods too, not seen thriva