Equations numbering with the subequations environment

I’m trying to fix some equations numbering, using the subequations environment. Here’s a MWE showing my issue:

documentclass[11pt,letterpaper,twoside]{book}
usepackage[T1]{fontenc}
usepackage{lmodern}
usepackage[total={6in,10in},left=1.5in,top=0.5in,includehead,includefoot]{geometry}
usepackage{microtype}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{mathtools}
usepackage{tensor}

begin{document}

Blabla:
    begin{subequations}
    begin{gather}
    begin{align}
        x &= a + b, 
        y &= c , d.
    intertext{Some usefull blablabla:}
        u &= t + z, 
        v &= r , s,
    end{align}
    
        f(x) = g(x).
end{gather}
end{subequations}

end{document}

Preview, with the numbering I need to achieve shown in red:

All math items need to be aligned with their equal sign, except the last item which is too long and must be centered (hence the gather environment). The intertext is essential, since I need the four first equations to be aligned for aesthetical reasons. The trouble is that I also need the first two equations to be sub-numbered (1a, 1b), and the rest to be sub-numbered with new numbers (2a, 2b, 2c). How can I do that?