Academia has convinced you of things that are not true.
Your post has a strong air of imposter syndrome about it, but let's assume that you are largely correct. You managed to qualify for a PhD with an undergrad record that was not truly excellent, but you still managed to qualify for a PhD. Most undergrad records are not sufficient to do that. GPA of 3.4 isn't "mediocre student" unless your school goes in for some severe grade inflation. You stumbled through your PhD in 6 years - longer than you would have liked - but you still managed to get through it. Many who go for PhDs drop out somewhere along the way. You would have gotten thrown out at any other university... but you got your degree from a top 50 school? No. No, that does not compute. You got your degree from a top 50 school.
From the facts on the ground, you do have good qualifications. Further, your core is a lot stronger than you think. You state the it taking too long was "mostly your fault". Probably you're describing running out of psychological resources of various sorts and therefore not being able to utterly power through at every point. Yeah... that's the kind of marathon a PhD is. The fact that you had to pick up a bunch of this stuff from self-study actually reflects well on you. It's additional difficulty that you had to plow through on your way up, and your willingness and ability to do so are also good traits.
Mostly? You sound seriously burned out. You spent all that you could of your psychological resources in order to make it across the finish line, and then you went looking and dug deeper and found more, and spent those too. You finally got your degree (go you!) and now the bill is coming due. This is normal. It sucks, but it's normal. Coronavirus is almost certainly making it suck worse, because Coronavirus makes basically everything suck worse for everybody. What you need is rest. Rest, take care of yourself, re-establish a degree of a social life, reconnect with the things that you enjoy, and generally take care of yourself. Nourish your soul, and allow yourself to heal. You have a solid position until April. There's time in there to recover, and you should take it. Once you've had some time to recover, I suspect that you'll find the situation is not nearly so terrible... and there are places in industry where "I have a PhD" is a serious qualification all by itself.
Actually, I suspect that that's the final part of your problem. You've been aiming at the top of your target range at every level up until now. You got into a PhD program at a top 50 school, after all. That's how you do that thing. Now you've hit a point where you look at the top of the target range for "graduated a top 50 school with a PhD" and you don't think you can hack it. You know? You're probably right about that... but that's also normal. Every time you hit that top bracket, the people around you get smarter and the challenges greater. There's no shame in being the middle of the best of the best rather than the best of the best of the best. Once you recover emotionally, start looking at positions a notch or two down from where you were looking before. I suspect that you'll be able to find places where you can do things you enjoy and contribute meaningfully without burning yourself out in the same way, and places that will be delighted to get you... especially with your demonstrated willingness and ability to self-study when necessary.
One little anecdote: I have an aunt who ran one of the shared biochem labs at her university for many years. It was a position that only required a Masters, but she had a PhD. Still, they were happy to have her there, and happy with the moderate amount of low-pressure experimentation and publishing that she did with spare time and resources. She, in turn, rather enjoyed being able to talk with the grad students without being terribly intimidating, but also pull out her doctorate when one of them was insufficiently respectful. There's lots of positions out there that you're plenty qualified for, or even overqualified for. If you're willing to put them in your view, I suspect that things will not look so dire.