You've read about both approaches: changing thinking and changing behaviour. You understand you can't do both, so now you need to choose one.
These questions should help.
Do you have a deadline?
A deadline may be a legal requirement, an audit or a product launch. If something is forcing your hand, behaviour change is probably your answer. You don't have time to wait for thinking to change. You need people doing things differently now.
Do you have the authority to enforce a process change?
Behaviour change only works if the bar you set is real and consistent. If you can't really make accessibility part of the definition of done, changing thinking might be more realistic.
How big is your team?
Real changes in thinking happen through close contact, real conversations and shared experiences. That's harder to engineer across a large or distributed team. Behaviour change scales better because it lives in the process, not in people's heads.
Have you tried before?
If you've already tried training people and it didn't stick, stop trying to change thinking. Change the process instead. If you've added accessibility to your workflow and people kept ignoring it, the process isn't the problem. The thinking might need work first.
What do you want long-term?
Compliance or culture? Behaviour gets you compliance faster. Thinking, done well, builds something that lasts. Neither is wrong.
Answer these honestly and the right choice usually becomes obvious.