Mobile redirect in nginx with query string

I’m trying to redirect mobile visitors to AMP pages with nginx.
I’ve basically using the script @ https://gist.github.com/dangayle/2919206 with recommended modifications, plus elements from the answer @ add a URL suffix path (for mobile AMP) without causing a redirect loop in nginx_

The main difference here is that I’m not trying to redirect mobile users to a mobile sub-domain, and instead just add a query string (“?amp=1”):

That’s the code in the HTTP block for nginx.conf:

    default 0;

    "~* (android|bbd+|meego).+mobile|avantgo|bada/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)/|plucker|pocket|psp|series(4|6)0|symbian|treo|up.(browser|link)|vodafone|wap|windows ce|xda|xiino" 1;

    "~* ^(1207|6310|6590|3gso|4thp|50(1-6)i|770s|802s|a wa|abac|ac(er|oo|s-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw-(n|u)|c55/|capi|ccwa|cdm-|cell|chtm|cldc|cmd-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc-s|devi|dica|dmob|do(c|p)o|ds(12|-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez((4-7)0|os|wa|ze)|fetc|fly(-|_)|g1 u|g560|gene|gf-5|g-mo|go(.w|od)|gr(ad|un)|haie|hcit|hd-(m|p|t)|hei-|hi(pt|ta)|hp( i|ip)|hs-c|ht(c(-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i-(20|go|ma)|i230|iac( |-|/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |/)|klon|kpt |kwc-|kyo(c|k)|le(no|xi)|lg( g|/(k|l|u)|50|54|-(a-w))|libw|lynx|m1-w|m3ga|m50/|ma(te|ui|xo)|mc(01|21|ca)|m-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10(0-2)|n20(2-3)|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|-((1-8)|c))|phil|pire|pl(ay|uc)|pn-2|po(ck|rt|se)|prox|psio|pt-g|qa-a|qc(07|12|21|32|60|-(2-7)|i-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55/|sa(ge|ma|mm|ms|ny|va)|sc(01|h-|oo|p-)|sdk/|se(c(-|0|1)|47|mc|nd|ri)|sgh-|shar|sie(-|m)|sk-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h-|v-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl-|tdg-|tel(i|m)|tim-|t-mo|to(pl|sh)|ts(70|m-|m3|m5)|tx-9|up(.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5(0-3)|-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas-|your|zeto|zte-)" 1;
}

and the remaining code in location /:

set $mobile_rewrite do_not_perform;

if ($mobile_agent = 1){
    set $mobile_rewrite perform;
}

if ($mobile_agent = 1){
    set $mobile_rewrite perform;
}

#check if query arg = yes (example.com?mobile=yes), set variable
if ($arg_mobile = 'yes') { 
    add_header Set-Cookie mobile=yes;
    set $mobile_rewrite perform;
}

#check if cookie mobile=yes, set variable
if ($cookie_mobile = 'yes') { 
    set $mobile_rewrite perform;
}

#check if cookie mobile=no, break
if ($cookie_mobile = 'no') { 
    set $mobile_rewrite do_not_perform;
}

#check if query arg = no (example.com?mobile=no), break
if ($arg_mobile = 'no') { 
    add_header Set-Cookie mobile=no;
}

#if $mobile_rewrite = perform, do the redirect
if ($mobile_rewrite = perform) {
    rewrite ^.*(?<!/?amp=1)$ $scheme://www.cnx-software.es$request_uri?amp=1 break;
}

I still get troubles with redirects stuck in a loop. For example: https://www.cnx-software.es/2021/08/31/arm-china-ahora-es-totalmente-independiente-de-arm/?amp=1?amp=1&amp=1?amp=1&amp=1?amp=1&amp=1?amp=1&amp=1?amp=1&amp=1…

The problem is probably the rewrite line, but I have been able to pinpoint the exact issue.