19.04.21 is the earliest date when they expect you back for the application to the 1-year Extension of Stay Permit based on marriage to a Thai. That's 30 days before your admitted stay ends. They expect you to show the financial proof needed for the 1-year extension. Nothing said that you can't apply for the 60-day Covid9 extension, or the 60-days extension based on family visit. You can, but these 60 days are not the right way to obtain a long-term stay in Thailand. Only the 1-year EOS based on marriage is. What goes for the documents for the Covid19 extension , you can use the search function of this group to find it "covid-19 extension"
90 days after your entry into Thailand. Check the triangular red stamp you received upon entry. If you haven't been issued a 90-day report slip neither by the 16th of February nor by the 16 th of march, the entry date still shows that you need to report 90 days after this date
and then there is this, the 30 days "normal" extension will be counted on top of the initial 45 days, whilst the 60-days covid19 extension will be counted from the day you apply for it
you will be able to extend the visa-exempt for 30 more days, for 1900 THB, at the Immigration just shortly before the 45 days admitted stay run out. Most Immigration Offices want you to use the 30-days extension before they allow you to hop onto the 60-days covid19-extension. Some offices let you have the 60 days right away. It depends on the office
you also have to meet the financial requirements for a 1-year extension of stay permit out of an O/A Visa. These are either 800,000 THB on a bank account in your sole name, seasoned for 2 months on the date of your application, or proof of a monthly income of a minimum of 65,000 THB. Seeing you are from the UK, the later option would require a full year bank statement of incoming from abroad 65,000 every month for the recent 12 months (because your embassy does not issue a letter of income proof any more). That and the mandatory Thai Health Insurance showing cover for the whole duration of the year you will receive the extension for, and you will be fine to go (plus passport, TM30 receipt, 90-days report receipt, Bank book, etc)