If you predominantly shoot JPEG and already have an XT5, I would buy a 16, 18, or 23mm lens to go with that. Spending significantly more on a Q3 would be a waste of money.
I agree with you on the XT5 advice. If he already has it, that is already light and minimalistic and with a small zoom he has a lot more versatility.
I had many x100 series cameras for many years when I shot Fuji X, but then I leaped to Fuji GFX medium format with the Q2 and then Q3 as a compliment to that heavy gear.
I love it.
But anytime someone says any Leica product is a "waste of money" I chuckle.
The Q3 is a waste of money only if you can't afford it, and if you can't afford it, don't even think about it.
I love the Q3 because it is beautiful, very compact, Leica build quality, and that great Summilux 28 mm lens. (Note: I like the Q2/3 size way better than the smaller X100 series - the Q3 is much better in hand).
The IQ of the APSC sensor is probably just fine for the OP but I prefer FF over APSC now and also prefer MF over FF because I am a resolution IQ junkie on big 6K monitors and like the latitude in post one gets as you climb the sensor size chart. But again, that is not an issue for the OP.
I also prefer 28mm FF of the Q2/3 than the 35mm FF equivalent) X100 vi for a fixed lens camera because I like wider if I have only that one fixed lens.
Also, I don't like the conversion of that fixed lens or any kind of cropping options. If you need that, get a zoom on your XT-5. Treat the X100 vi for what it is. a 35mm equivalent lens. (Many people prefer 35mm to 28mm if you have a fixed lens, but I don't.)