Thoughts from a 36yo engineer who has decided to transition away from IT:
Specialising in the areas you mention is a big bet, as it implies that the product / architectural approach will remain constant as time goes on. Except for enterprisey institutions like Oracle, SQL Server, or big ERP systems, this isn't really a tenable strategy as the rest of the industry carries on innovating.
Rather than specialising, the modern approach to longevity is learning. You can still carve out your preferred area (e.g. backend), but you must be prepared to move with the tides. You don't have to be an early adopter of every new hot, prototype tech that comes about, as that takes a lot of energy and can be frustrating. Instead, consult industry publications like InfoQ which cover more long-term patterns in the field, and use those to inform your choices of further investigation. This will expose you to actual use in the field, by other companies (e.g. how many seem to be using it, and what common failures are experienced). For example, microservices has been around since about 2010, and was borne (IMO) from Service-Oriented Architecture which was around since about 2005. If you take notice of the rise of cloud technology, you would be able to see the trend towards separating applications and separating processing.
Cloud and microservices, regardless of their foibles or unsuitability, are often a reality as the tech industry moves in a way that is not really very wise, and things are often rediscovered in different ways. As an alternative to embracing this "move with the tide" approach wholeheartedly, you could position yourself as a wise consultant-type figure (not necessarily an actual consultant, just an employee who has wisdom), and push for the right approach for a given situation. I would still keep abreast of trends though.
Something that has worked for me is doing university / polytechnic courses whilst working. This will add interesting flavour and variety for your mind to work on during the day, and you might discover a new interesting field (for me, it was people and HR, the role I'm now transitioning into). If you want to do this, I'd recommend the book: The HBR Guide to Changing Your Career (has advice on how to gradually incorporate more of what you want to be doing into your day job, safely).
Best of luck in your quest!
[Edit] You mention "good old RDBMS" which suggests a strong affinity to things in your comfort zone. I'd suggest Googling "Growth Mindset" and "Immunity to change" as tools to help you recover your curiosity. Remember: Don't focus on others already knowing a subject better than you, think about what you know now and improving that knowledge exclusively. Otherwise, you will always persuade yourself out of learning something.