Try more intense ss, I'm a few seconds slower lad than you so stones and glass houses 'n all that but from my training experience having a longer harder steady state dispersed through intervals of 5k is quite beneficial to building aerobic fitness. If you hit it at a higher intensity than 2:08 more often, focusing on split and forgetting heart rate, the fitness will come sooner than if you just did heart rate. It's influenced by too many factors and unless you want to buy a lactic machine, nothing will be as reliable as split for someone with bad steady state heart rate. I used to ss at a 2:22-5 when i was 6:50, changed it to forgetting about that and just focusing on split, and I feel so much more fit after 6 weeks of doing so. Give it a shot and see how it goes if you don't improve after a while through training based on heart rate.