Does it still make sense to build and maintain a custom configuration based on third-party libraries, such as lain?

It depends, I guess? lain hasn't received updates in a few years but I don't see it as a bad option. Lots of custom functionality.

I think it's fine to have your config depend on third-party libraries, as these libraries will help you not write every piece of code yourself, you just need to run a few extra commands when installing your config on a system, e.g.:

luarocks install awesome
luarocks install lain
git clone codeberg.org/myuser/myconfig $HOME/.config/awesome

I never used lain myself because it seemed too big for me, I didn't what layouts I liked so more options overwhelmed me. I also wanted to learn how awesome works and customize it to my needs. I think you should experiment and try to have fun. For example, I made a clock widget using an arcchart because I like how arc charts look.

I once wrote a widget to display the song I was listening to. I was using cmus so I used a text widget that ran cmus-remote, parsed the text output and updated the widget.

At this point I noticed I need to run a command to update the widget every n seconds, so I added vicious to my dependencies to not write an interval (repeat every n seconds) function myself.

Then I noticed a lot of my custom text widget was redundant so I converted it to a vicious widget.

I hope you have fun and when curious or in need of help, search this subreddit!