in theory, i'm not opposed to kara having a civilian as her love interest, but so far, in all of supergirl media, william dey is the only one i've loved and think could be a long-term partner for kara.
william is fucking insane in the best way possible. he's insane in the same way kara is. william is a force of journalistic nature, the same way lois is. what makes lois so special makes william special too. his best friend gets murdered, so he goes on a crusade to get him justice. he moves to another continent just because he's chasing a story. he can't get close to anyone because he can't be fully honest with them. he wants nothing more than to get justice for russell, but not at the expense of others getting hurt. no one should get hurt because of him. he wants to protect everyone. and if anyone has to get hurt, it should be him, because this is his mission. this is literally kara's way of thinking when it comes to being supergirl.
william is also an immigrant, just like kara. and that's such an interesting aspect to explore. he's the only human love interest kara has who isn't american. he can relate to her in ways others can’t. he can understand her fully. and that’s something kara deeply wants in a romantic partner.
kara and william also share the same morals. they're like-minded people who can still challenge each other and bring out the best in each other. kara has a more emotional approach to her stories, while william’s is more logical. neither is better, it just depends on the situation whose method works best. also, both of them are polyglots, so there’s that too.
all in all, i think a civilian love interest could be a great fit for kara, as long as you make them truly relate to her.