Here's the relevant Atomic Rockets page.

If you're sticking with NTRs, the main way to get more power out is to increase the reactor temperature. That inevitably leads you to liquid-core NTRs, and even gas-core (though these are either open-cycle, or have an operational lifetime measured in minutes).

But most NTR designs center around higher exhaust velocity (so, higher ISP), instead of higer thrust. This is because, unless you have a torch drive, you must choose between high thrust (by throwing more mass out the back) and high ISP (by throwing less, but faster).

Since high thrust needs more reaction mass, it invokes the tyranny of the rocket equation, so we prefer high ISP, and just take longer to get places. But ISP is limited by how hot/fast you can make the propellant (chemical engines max out at roughly 600 seconds). That's why we even want to use NTRs in the first place.

Also, TWR is only really relevant on/near a planet, and you do not want to be using powerfull nuclear engines anywhere near that. Regular NTRs should be fine, but unless you're thinking of taking off or landing with them, don't worry about the TWR.