I have 10 years experience primarily into UI development, what tech stack do you recommend to upgrade next as per current job opportunities?

Nodejs will be easier to learn and it will be easily able to transition you into backend role. Python, even though most popular right now, may be a bit difficult to learn and transition to, coming from UI background, and UI -> Python transition may even seem abrupt to folks if they observe this in your resume. That said, the benefit of Python is, not only would it provide you a backend background, it will also place you closer to all the AI stuffs going around these days. So while difficult, more effort taking and a bit risky path to take for you; it can certainly pay nicely if you put in really dedicate efforts. P.S.- C# should not even have been in the option! 😄