Like your friend, it depends, lol. I work at an agency that starts many projects each year. With that, we're lucky enough to greenfield almost all of them and use the latest versions of things. So, we wait very little. However, I think more places are not like us and wait longer. It takes time to upgrade and time equals money ultimately.
One nice thing about Rails is the stability of the framework. Yes there are changes with a major version bump, however, the large majority remains the same. Solving problems in Rails 5 will not severely hamper you if you were to end up working in Rails 6 in the future. You'll be knowledgeable about most of the framework and the new code might not even be using any Rails 6 specific things.
In my opinion, keep doing what you're doing. Finish your project, then upgrade it. You'll learn some things along the way.
Now that both you and Ben both advised the same thing, I'll definitely take that route. Thanks for the confidence booster, Seth
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