First use Google Chrome then go to friend invites page:
https://gtm.steamproxy.vip/my/friends/pending

Press F12 and replace the third row "level=100" with a decent number.
For example, if you want to decline all invites whose level below 50 just type level=50.
Then go to the 'Console' and paste codes and hit enter.

a= document.getElementById('search_results');
b = a.firstElementChild;
level=100;
while(b){
if(b.id != "search_results_empty"){
d=b.children[2].children[0].children[0].children[0].children[0].innerText;
if(d<level){
e=b.children[3].children[2].href.replaceAll("%20", " ");
eval(e);
}
}
b= b.nextElementSibling;
setTimeout(null,100);
}

Use Tampermonkey to auto-decline low level friend invites.