Rivnuts for M5 screws have a 7 or 7.5mm OD, while you'd use a 1/4" (6.35mm) drill for a brazed in boss, so yes - a hole made for a rivnut will be too big to braze a boss into later. You could make a custom braze-on boss, but it's probably not worth the hassle. Correctly installed rivnuts work very well, and as has been pointed out, you'd have to figure out how to get them out of the tube should you choose to remove them later. How difficult that is depends on how big your vent holes are!
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