Or should I just make a 8/3 100ft cord? I know its expensive, but I want it done right and safe.

You can make a cord. Note that cordage counts the ground so you need 8-4 (unlike cable which doesn't count the ground).

You can't run cord through the walls/structure, so unless the transfer switch is mounted exterior (or in the garage or someplace you can go through the door), you'll need to mount a inlet on the house and run approved interior wiring to the transfer unit.

Can I get away with 8/4 on the inside from the transfer switch to the inlet box and just do the rest (50ft) with a pre made cable of 10/4? Would this help with voltage drop vs doing everything in 10/4. Thanks

I think you mean 8/3. Cables don't count the ground again (cords do).

Yes, you can use cable inside the house (provided it is rated for interior use, I'm assuming you're talking about something like NM). Going up a wire size when you've got high currents over longer runs isn't a bad idea.

Your correct 8/3. I'm going to just make a straight 85-100 ft cable and run it directly from the gen to the transfer switch.