In some areas of IT these days you don't get jobs solely through responding to adverts and applying. The jobs are advertised and the interviews happen but there's a practical element to the recruitment process that I suspect you aren't participating in - which is about your online presence.
In software development and support work astonishing amounts of what gets done emerges from exchanges in online forums that are rather like mumsnet for mega-geeks. Think of a few kinds of questions that people more junior than the posts you are looking for are likely to ask along the lines of how to I make (this bit of code) behave as I want it to in (this kind of circumstance) and google it - you will get hits from various websites where knowledgeable gurus are just as enthusiastic about offering advice as a CF Parking-On-My-Drive thread in AIBU.
On these forums, users get upvoted and acquire kudos points for having supplied the best advice and they build up a reputation for how reliable their advice is. And unlike mumsnet, people don't hide their online identities but their CV will say things like "Active on Substack as Xoraxial56" - and anyone can then go and look at their profile.
When a company is recruiting into a role that requires experience of this type then although this never makes it to the HR-approved specification, the recruiter is going to be looking for evidence of an online profile of activity to find out whether you have a positive reputation to go with your years of experience.
Also a lot of roles in IT don't follow a structured pattern that a vacancy opens up because a particular staff member left, and therefore the organisation advertises the vacancy and fills the role. Team structures flow more dynamically than this - it's more likely that when a vacancy opens up the roles and responsibilities of the person who left will get absorbed into the roles of other team members who are looking fir more responsibilities and new challenges, and an entirely different vacancy will be created which happens to be a good fit for someone that the senior team are aware is job hunting at the moment. The role may well be advertised but they already have their eye on a likely candidate before the job description is published.
To get your foot in the door for those kinds of jobs you need to make yourself known to people inside the kind of organisations you have your eye on. Ask the right people if they can spare you some time to tell you a but more about what they do, and find out whether your own skill set is likely to be something that will benefit their organisation. If they like you and you are able to impress them, then a vacancy that is a you-shaped hole can later get created and you'll be invited to apply. Attend events in your area of expertise, and contribute actively. Listen to other contributors to find out what kinds of projects they are working on and think how you could make a difference. Make sure people know how capable you are by actively demonstrating that capability.
The unfortunate thing is that the people you are competing with have already been doing a lot of this for the last 10 years while accruing their own experience and expertise. It's rather difficult to start from scratch, but not impossible.