It's possible that a InfiniiVision 1000 X-Series scope could work for you, it does gain and phase up to 20 MHz if you get a "G" model.
This can be a challenge on a scope because of the low level signal that is typically buried deep in the noise of the switching elements. The latest software appears to be filtering/averaging, but I cannot see what it is doing in the background. The dynamic range is considerably better than the original application.
I have been able to rough it manually using averaging and filter math to get what I needed. I manually step through the frequencies and measure the gain and phase with the scopes measurement tools. In general, you are looking for two distinct points - the phase margin and the gain margin which means you
don't have to analyze in 1hz steps. This is rather slow and somewhat limiting - but cheap. Forum member Robert Bolanos demonstrates this in his YouTube video posted a few replies up.
The next step I took was to use a differential amplifier Preamble/LeCroy DA1855 to better see and filter the low level signal. That requires some time compensation to get an accurate phase measurement but the benefit is that the low-level signal is gained to a point where the scope is not on the edge of its front end performance. This is not critical, but helpful. The main use I have for my 1855A differential amplifiers is to measure current shunts.
Next for me was getting a VNA. I found an HP 3577A that was originally used in the Paleolithic period by cavemen trying to analyze the energy transfer at the contact point of two sticks being rubbed together. I searched for quite a while and ended up with a mildly broken one for $200 plus about $75 to fix it. I added a Jensen video isolation xfrmr VB-1BB that was another $20 off of eBay. I am still learning how to get this setup and deliver trustworthy measurements. So far, it seems rather fantastic relative to my previous efforts. It goes down to 5hz and has 1hz RBW, averaging, etc.....lots of relevant features for FRA. The alternative I was looking at was suggested by power integrity guru Steve Sandler was a Keysight E5061B with low-frequency option. This is an amazing tool - but with an appropriately amazing price. The 3577A used as an FRA is a great bang for the buck as long as you have a bench big enough and strong enough to hold it.