1. You should apply for Indefinite Leave to Remain based on your five years continuous residence in the UK. So long as you had a visa which allowed you to live and work in the UK, you can apply for Indefinite Leave to Remain. Once you have Indefinite Leave to Remain for a period of one year, you can then apply for UK citizenship.
2. Here is a link to the UK Government website which sets out how to apply for Indefinite Leave to Remain https://www.gov.uk/settle-in-the-uk.
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4. You can go direct to citizenship if you wish simply by getting a Permanent Residence card and holding this for a year. Then at the end of the year, you can make a UK citizenship application. Here is a link to the form for applying for the Permanent Residence card https://www.gov.uk/government/publications/apply-for-a-document-certifying-permanent-residence-or-permanent-residence-card-form-eea-pr. This avoids applying for Indefinite Leave to Remain and you simply apply for UK citizenship after a one year period.
5. I have read your work and pension history. However, you can aim to get citizenship after one year holding the Permanent Residence card. This is the most direct route to permanently staying in the UK.