Do get a HRM.
I think you'll learn over time why the criticism of HR is valid - it frequently de-couples from perceived effort, especially if you're fatigued, dehydrated, short on sleep, or a variety of other reasons.
For example, yesterday I went out to do 2x20, using my HR as threshold guide. But as soon as I got on the bike I knew I hadn't fully recovered from the weekend rides. My legs told me, and my HR confirmed it when I couldn't get it up into the target range. So I just rolled around for a while... and later my legs started feeling better and I was able to knock out some efforts, bu tmy pulse never did come up into target range.
Anyway, you'll learn a lot by watching HR.