But how do we draw the line between this and buying property to rent to others? Is being a landlord acceptable as a communist if we realize we are forced to participate in the system?

Directly and purposefully shifting from Proletarian to Bourgeoisie is probably not a good move for a Communist. Materially, one could become a Landlord and intentionally take no profit, which would benefit the Proletariat, but would merely be charity work within a dying system. Instead, joining a Renter's Union or owning your own home outright is better, the former helping all of your local proletarians and building up power.

When it comes to investing, it's important to think of aggregates. A Proletarian clawing back some of the wealth stolen from all of the Proletariat does not mean said Proletarian has taken back more Value than they have had taken from them in the first place.

Finally, we must consider what it means to behave Morally in Capitalism. Yes, we all can and should boycott the worst of the system, but unfortunately, we are forced to engage with it. The only path out is to organize, no amount of self-flaggelation and rejection of wealth will change this process. Moralism is a dead-end, Materially.

It's a complicated subject philosophically, so you must consider what your goals are, and why. Communism isn't about being the most morally upstanding victim of Capitalism, but a path to eliminate and replace the system altogether.