Honestly I think double boiler/HX machines are a waste unless you want to do a lot of steaming. Of course arguments can be made regarding thermal mass, temp stability, etc. I've owned and/or used some really good machines and based on extracting tens of thousands of doubles over the years I see the higher end machines being fine if you want to obtain consistency with less effort. Lower end machines (single boiler) will easily extract on par with them, just takes more effort to do so consistently. If you can control brew temp/pressure you have espresso. Doesn't matter what machine it dribbles out of.
How is the espresso out of your Gaggia? 2-3 degrees drop from start to finish is what I would call acceptable. Most people likely won't taste the difference a few degrees makes. Have you tried using something like a type K probe connected to a digital temp meter for temp readings? I use one and snake it up through the spout so it rests right under the basket, then do some flushing to see where my temps are. You could do a 2 oz. flush and take note of temp readings throughout. May not be super scientific, but good enough for me. Some tend to overthink/complicate things and you end up chasing numbers/changing variables instead of relying on instinct, taste, etc.
My daily driver is a stock Silvia and contrary to popular belief it's quite the machine when you figure out a good routine on temp management. Once you do it's very consistent and very capable. I still don't see the need to PID it.
Sounds like you're likely maxed out on what you can do with the small boiler setup. The way I look at it is Gaggia designed the aluminum/brass hybrid boiler with high power external elements for a reason. It has been in use for a long time. Same with the Silvia... Most think it's ancient, but if you look under the hood you will see lots of thermal mass in the form of a heavy brass boiler, offset group and portafilter. Once it's warmed up it is very temp stable. Things like that are built to function well and do so for a long time. No need to fix what isn't broken. Also, one thing I find awesome with the Silvia layout is that all water/steam connections are designed/implemented that in case anything ever leaks it will fall straight down to the drip tray. No wiring, switches, pump, etc. should get wet.
