For those who love God all things work together for good

[TonyChanYT]

Romans 8:

28 And we know that for those who** love** God all things work together for good, for those who are called according to his purpose. 29For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. 30And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified.

Paul was being quite specific. He was talking about true believers.

Is God’s promise to work all things out for the good of those who love Him individually or merely collectively/in general?

The promise is true for every single true believer.

And if it’s a promise to individual Christians, does it just mean they’ll end up in Heaven?

Right, they will receive glorified bodies in heaven even after 'bad' things happen to them. God knows exactly what happens to them. Luke 12:

7 Indeed, the very hairs of your head are all numbered. Don’t be afraid; you are worth more than many sparrows.

Don't focus on the external circumstances; focus on God who foreknew you.