Just a quick Q, do the temporary hitpoints granted by the Heroism spell stack on their own? I know the PHB rules state that temp hitpoints from different sources don't stack but I'm confused about Heroism. Thoughts?

The Player's Handbook never says "different sources"; here's the actual quote:

If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones.

That applies regardless of where the temporary hit points are coming from, unless you're explicitly told otherwise.

Temporary hit points never stack - recasting Heroism replaces your temp hit points unless your current temp hit points are higher. From https://www.dndbeyond.com/compendium/rules/phb/combat#TemporaryHitPoints

Healing can't restore temporary hit points, and they can't be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22.

That is one way to look at the temporary hit points granted by the spell. There’s also an alternative. What would you say if I affirm that the temporary hit points that you receive by the Heroism Spell are granted over the duration of the spell (a period of 10 rounds with Concentration) at the rate of « your spellcasting ability modifier » each round. Those are not new temporary hit points being given from different source but the hit points coming gradually into effect from a unique source, the heroism spell.

As mentioned above, temporary hit points do not stack, regardless of the source. Heroism, that grants you a few temporary hit points each turn, effectively "resets" them if they're gone - it's practically preventing a small amount of damage each turn.