Best browser aside, how do I solve the problem in Safari? Thanks.
Two bits of advice. First, get over it. Second, enable the Develop Menu and try different User Agent options. I am not being snarky or deprecating. I like Safari, and will use a browser that works for the web site/portal that suits my needs. I also use both Mac SSBs Fluid and epichrome, I'm using an epichrome SSB for this web portal.
I find that some web sites work poorly in Safari, but fine in Chrome. Then, I'll launch Safari and change the latest User Agent to the Mac version Chrome embedded in Safari and I'll find that the web site that didn't work well in Safari with Safari as the User Agent works perfectly in Safari with Chrome as the User Agent - this leads me to believe that web site admins are purposefully blocking Safari because they can. I'll set up a SSB in Fluid and it won't work well until I change its User Agent to Chrome or FF, again reinforcing my perspective that some domain admins are blocking Safari.
On my Mac, in the User Account I use to surf the web I have the following installed - Safari, Safari Tech Preview, FF, Chrome, Fluid, Vivaldi, Opera, and SSBs that were created by either epichrome or Fluid (the paid version). I've learned to just roll with whatever works and deal with the rest. If that's too much beyond the "why doesn't Safari just work" perspective for you, move on from this post...
