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I use a round polished wood tool like this one, which is normally used on leather. I just jam it into each buttonhole once from both directions, which greatly loosens up the buttonholes on heavyweight denims. I do that once before the soak, and then if needed, again after the soak.
I like to use a round polished wood tool to slightly expand and soften up the stiff denim around tight button holes.
Some people will use a sharpie, screwdriver, or needlenose pliers to loosen up their buttonholes.
If that still doesn't work, I've taken a pair of sturdy scissors and snipped the too-small buttonhole slit to be a slightly wider slit.
I do that slit-cutting at the end of the buttonhole slit which is the opposite end from where the button pulls on the buttonhole when fastened, so that it doesn't really matter that much that I'm actually cutting into the buttonhole stitching and denim a tiny ways, maybe 1/32" (0.8mm) to 1/16" (1.6mm) into them.
I can understand why some people are apprehensive about using this buttonhole-cutting approach, but it works great for me without affecting the integrity of the jeans at all, if done carefully and minimally.
Glad to hear everything worked out well for you! Now you have a good baseline set of measurements on your new pair for comparison with any future pairs.
If you have a hard time getting the buttons into the buttonholes, you can push a round smooth instrument into the buttonholes to help loosen up the denim around the holes, such as a sharpie pen or a leather burnishing tool like this one that I got for myself (pic of tool in buttonhole). Some people have even used a screwdriver.
Because raw denim is so stiff and rigid, I like to machine wash and dry my jeans first before wearing them to help soften them up a tiny bit. If you are aiming for high contrast fades, one single wash and dry will remove very, very little indigo dye, even though the soak/wash water does look very dark sometimes.
Enjoy your new Momos, they are a great brand.
Lighter weight denims, and heavyweight denims too, can also have buttonholes which are sewn too small, and you can use that same approach to push something thru the buttonhole to help stretch those buttonholes out.
Another approach I've taken for buttonholes which were still too small to easily get a button thru after stretching them out with an implement, is to take a pair of sturdy scissors and snip the too-small buttonhole slit to be a slightly wider slit. I do that slit-cutting at the end of the slit which is the opposite end from where the button pulls on the buttonhole when fastened, so that it doesn't really matter that much that I'm actually cutting into the buttonhole stitching and denim a tiny ways, maybe 1/32" (0.8mm) to 1/16" (1.6mm) into them.
For just normal stretching out of stiff-denim buttonholes, I learned about a leather burnishing tool from Isaac of Pigeon Tree Crafting (u/pigeontreecrafting) in his Tanuki Redcast review in this sub, which works great for stretching out buttonholes.
The burnishing tool which I bought is this one. And I was glad I had it just this week, when I needed it to stretch out the buttonholes on my Oni 20oz double-indigo Secret Denim jeans because the bottonholes were too stiff and unwieldy to get buttons thru them without damaging my fingers and thumbs in the process.
The buttons now slide in and out of my Oni 20oz jeans super easy, like slicing butter.