An interview should not take 2.5 years. First you write an online exam, and if you pass then they put you in a queue for up to a year, during which time they might bring you in for a gruelling (and highly representative) set of testing. If you pass the tests, within 30 days you'll have a 1-1 phone interview with a retired controller (this personal interview is structured, recorded, and highly representative). Then, they'll put you in a queue for up to 1.5 years, during which time they could phone at any time and say "hi, we're starting a training course, if you are available we want you to pay the expenses to come to us for a three-on-one interview". If you pass that, they'll send you a pile of textbooks and you'll need to self-study for 50 hours (actually, 100 hours), for free and on your own time, and there will be an entry examination when the course starts. I understand the reasons - ATC training is gruelling and the ATC job is exacting, so careful selection is required (otherwise candidates will drop out, or be told to drop out, at an excessive rate). But the duration is excessive. Small wonder there is a severe shortage of ATC. I am still in the queue. No offer. Yet. Glassdoor only allows a maximum duration of 12 months. So far it has been 24 months.

The whole process is just the same as they have mentioned in websites and emails they sent for invitations. Just prepare according to their instructions. The interview is a little bit serious. They want to find the person who fits the position.