A lot of small form factor units seem to lack much output filtering, but what you're describing probably could be done with a standard benchtop arb - something like a Siglent may actually be more cost effective than a different brand's RF source.
That said, you could look at the Holzworth HSM as well. They sort of specialize in low phase noise synthesizers, but their 1GHz source seems to be nominally -42dBc second harmonic, to +20dBm output, and USB controllable with an adapter (otherwise SPI and optionally ethernet).