I looked into this a couple months ago and external TB5 was still more expensive at 1-2 TB not sure about above, though.
Samsung 990PRO 2TB is $170 and Acasis T5 80Gbps is €300. So it makes sense to buy external for ≥ 2TB, more flexible as well :-)
For 1TB it makes more sense to buy built-in as you note above.