Where you put the wattmeter will depend on what you want to check. If you want to see the input tuning of the amp, then you’ll need it between the radio and amp. After setup, you’ll probably want to see the output of the amp, so it would then be last before the antenna. Or, like me, I don’t use a wattmeter at all after setup.
So, radio-amp-wattmeter like you said.
Placement of the low pass filter has been discussed a few times here with differing opinions.
One position is that the amp should only reproduce what goes in, so before would accomplish that.
If you decide to run it after the amp, just make sure it can handle the full power output. I tend to like things overrated for their duty.