A custom router + managed switch is a great way to learn. Studying the fundamentals is also good, but in my opinion it’s not as fun as setting up your own network and learning hands-on.
If you decide to go this route I highly reccomend taking regular backups of your config (and backup again before you change stuff). Part of learning involves breaking things - trust me you will break your network - and in networking that’s one of the best ways to learn. Backups will give you an easy way to restore to a known working configuration.