Caveat here that I only owned a spectrum analyzer once and it was an extremely old one.
If you want to be able to figure out what's going on with complex modulation schemes, a Gnuradio-capable SDR (or several) might be a good addition, and might even be able to do double duty as a sort of poor mans SA, unless you need a verifiable, calibrated device.
But many spectrum analyzer tasks aren't possible in realtime. You cant look at the entire frequency range it covers at once, without scripting.
So getting the big picture requires scripting it with external tools. Which is now quite easy and reveals a lot of info if you are willing to follow a workflow for extracting what you need from it, with a minimum of external components. (attenuators, cables, probes to track signals down) which you likely already have.
What I am saying is, its a good addition that doesn't cost much compared to some pro equipment and might be much more helpful in some situations. But others, less so.
For example, extremely broadband RFI might likely be much easier to find quickly with a portable spectrum analyzer.
You can see it on an SDR but figuring out its fundamental frequency and power vs. time characteristics requires running a sweep and recording the findings to a CSV file, and then visualizing it in a graphing program.
Summary: SDRs combined with data vis tools can give you a lot of info, and they are cheap, but its not 'plug and play'.