Harmony Bands don't give you more lifespan, but they do give you more starting Qi, which is critical. Strictly speaking, I don't think you have to run the Longevity activity in multiple Paragon cities, but you do have to gain enough familiarity (through any activity) in enough cities to have "studied" them in order to be able to raise Longevity further. The Mastery record shows how many.
I just myself got the Resonant Heart today, and there were three pieces to it. First I maxed out lifespan increases from Paragon cities, which I think gets you to around 385 years with all the previous lifespan boosts available. (It's certainly possible to stretch this a bit more with wilderness insights, but it's only a small amount for each one, so it's not critical. I'm at 396 at the moment.)
Then I had to grind Harmonized Settlement cultivation up near level 5000 (yes I ran automated lives for a few hours to do it), which got me up to Harmony band 12 (all affinities above 120 and a starting Qi of over 100 - I think it was 114). Then I used a Lightning Beast form for the Qi boost and spent most of a life training (not cultivating) in a Paragon city, which gathers more Qi faster than cultivation anywhere, according to my tests and skill levels.