I run BubbleUPnP on the same Win10 PC as the MC server and library, which has its music stored on a USB attached SSD. This server doubles as my main desktop PC for office apps and web browsing. My audio load is extremely low (playing ripped 44.1KHz CDs w/o DSP) - I rarely notice any audio degradation at all. The wifi network has 2 original Google Chromecast Audio devices (GA3A00147-A14-Z01), 2 MC client Win10 PCs, Android phone, and several other PCs. The system has been fairly reliable for MC 27 and now MC 28 (MinimServer also runs on all my MC PCs without interference, but is rarely used).
Your link in opening post has lots of details to check. At risk of stating the obvious, I'll make some "higher level" suggestions that work for me.
Don't overthink it - just try moving Bubble to your MC Server PC, as DJLegba suggested above. It's free, no risk, easily removed. I had an old single-user install for years, but just replaced it with latest 0.9-update40.
Check MC Options > Media Network > Advanced > all three DLNA boxes checked (Server, Renderer, Controller).
Do not run Bubble simultaneously on more than one computer. The MC clients can select the Chromecast zones seen by the MC server, but they do not run their own instances of Bubble while the server is running Bubble. I violated this rule for a while, and ended up with lots of confusion and very poor reliability.
Check Bubble server status on the MC server: RMB Bubble taskbar icon > Configure > Media Renderers > Create a DLNA renderer [checked] for each renderer. If renderers are not visible here, then restart Bubble. May be necessary to logout/login or restart server PC in worst cases. A single Bubble icon should always be among the task bar icons (either visible or in the hidden group). I also keep a Bubble startup shortcut on my desktop for easy restarting if taskbar icon is missing.
I have shortcuts in the windows shell startup folder (<Windows Start key> + R) which launch (1) JRService.exe, (2) javaw.exe for BubbleUPnP Server, and (3) MinimServer. These start automatically after windows login.
Also check that MC Media Server (JRService.exe) is running on MC Server PC. After login, it should always be among the task bar icons (either visible or in the hidden group). If it is not running for any reason, then firing up MC after login starts JRService.exe. (tools > options > Startup > Windows Startup > Run on Windows Startup: Media Server (...))
While you may not prefer the JRemote2 interface (personally I find it quite adequate), the app is really useful for a quick remote check of the MC Server status. The Player choices effectively show status of Bubble DLNA / MC Server combination.
Does your server go to sleep? If so, clients cannot connect of course! I find it useful to have a WOL app on my phone for those times, though JRemote2's WOL also works after a short discovery delay.
BTW, it's OK to install Android BubbleUPnP (which can access both MC and MinimServer libraries) along with JRemote2 on a phone. I occasionally use Android BubbleUPnP for those few times when JRemote2 fails (almost always traceable to Bubble failure on the server). It also provides alternative checks on the Chromecast Audio hardware. As usual, Google Home on Android is used initially to set up Chromecast Audios and check them if necessary.