You somehow have to slow down at Mars.

Either you need a lot of propellant for that which you also have to accelerate towards Mars in the first place, or you need a big heat shield.

If you insist on a modular system for the station, but you want to utilise a heatshield anyway because that's just far less mass than propellant, then you need to cluster your modules closer than on the ISS. You then can build the heat shield kinda like a surfboard.

As others have said: the ISS predominantly tough us to put as many systems on the inside for ease of maintenance. This and non-toxic cooling fluids. Because stuff will leak.

But when you engineer your way through all those steps and problems and try to optimise things you quickly realise you actually want something like Starship. Big volume, easy to build, integrated heat shield, enormous tanks.