Maybe you might prioritize what is essential. Things that are distinct enough that you only want the particular one chosen, or limited enough that options are not easily found.

For those items, continue to ping the internet every 3-6 weeks to check availability. If it looks like something may be changing, then try to find a site to purchase in advance that has a liberal return policy. Or you could comfortably purchase things that later would be easily sold on ebay (easy to ship and not breakable).

Ugh! I know what you mean. My original choices for lighting didn't appeal to me as much when I got things further along in the process. Now I am so glad that I didn't order them. I also loved a cup pull that I wanted to use but when my sample arrived it didn't look very nice IRL

I have changed so many things over the course of getting ready for this reno. We are now two days away from the start, my choices are set in stone and I am thinking it should all pull together pretty well. My electrician comes Tuesday and I still don't have my light for over the breakfast table ordered yet so I may have to get him to come back yet again!

I thought that I had nailed down finishes slightly over a year in advance of building, only to discover the tiles for two baths were discontinued, my stone slabs "disappeared" (after payment in full), and miscellaneous other items ceased to be available. A cascade of new decisions ensued. Either assume it will happen or find a way to take delivery and store safely now.

Actually I think I just found my vanity lighting for when I want complete illumination, made by a Co. that specializes in healthcare lighting.

Torture the internet for enough hours and something comes of it.

The one bathroom is almost completely monochromatic so if one element gets discontinued the whole thing could really fall apart.

Why not buy it all now? We had our whirlpool tub 5 years before install. Our kitchen cabinets 1 year.

Well, just regarding trouble finding dimmable fixtures, this is not something we've done, but have you considered retrofitting ones you like to hold dimmable LEDs?

Last house we bought things ahead of schedule only to be held up over a year due to parents health issues. Appliances sitting in their boxes past their warranty.
I didn't really care about not liking the light fixtures etc because the house was for a flip (we're still in it 7 years later...) However this house, currently building, is the forever place so I'd like to like the stuff I pick. We are looking at year two end of summer on this house and I don't like much of what I'd originally planned.
Yeah that and I found GW and I'm second guessing everything...

Periodically check on the items that you selected. If you find a great sale or something goes on clearance, buy it right then and there. That is what I did for our kitchen reno, which we just started. Many of the items that we needed we saved quite a bit on by sale shopping. We saved $2K alone on appliances, due to sales, rebates and price matching.

There are two thinks in my favor, I think. I like functional, Spartan bathrooms, and they are going to be small by necessity. The other is that the house is rather specific architecturally and there is very little leeway about what looks correct.

So the stuff I have picked is not anything too trend-driven and likely to disappear for that reason, but things change anyway, sometimes.