Ana, I'm currently in the process of getting a custom frame--a titanium Moots--with the hope that it will be "the bike.". Like you, I've only had one road bike and was unsure of whether I knew enough to venture into the world of custom. I'd note that I wanted a custom bike in part because standard geometries don't quite work work for me. I also decide to go custom because there are limited options out there in titanium frames.
Unlike you, I've worked extensively on my current bike with a fitter. He has a good understanding of the problems I've had with fit, what kind of riding I do and what we've tried to do to address my fit issues. it. I would note that some of my fit issues, however, are unrelated to the frame itself, but instead have to do with the "q-factor" (or width) of my crank.
I largely chose to go with Moots because my fitter works for the shop that serves as the Moots dealer here. I just felt more comfortable going that route. I don't think, it's necessary, however. A good builder should be able to work directly with you without a middleman. Still, I think having worked with a fitter who helped me understand my unique fit issues, which in turn gave me a better understanding of what issues we wanted the custom bike to solve. Some of those solutions were addressed by the frame itself; some by the components that we chose.
The part of the custom process that was harder for me was articulating how I wanted the bike to handle. IMO, this is where my limited frame of reference was most evident. Did I want a bike that was stable or responsive? Stiff or compliant? While I talked to my fitter at length about these things, it remains to be seen whether I chose wisely.
I still think it might be to your benefit to get a comprehensive fitting on your current bike, if only to make that bike better to ride. Once the bike is set up properly, it will give you a better frame of reference about fit if and when you go the custom route. It may also tell you whether you even need to go custom.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher