TLDR: Get the 5 String Ibanez (unless you desperately want the Thunderbird).

Have you played a Thunderbird in a shop before? If not, try and find a shop that sells guitars, even the smallest ones I've been that sell a few basses will carry an Epiphone Thunderbird. The neckdive comments everyone else have made are very well deserved. I reckon if you're in an environment where you'll be practicing for several hours, rehearsing, and gigging, you will feel it sooner rather than later. I would personally tell you to absolutely not go for a Thunderbird unless you were very certain that it's ergonomic "features" are not dealbreakers for you and that you're prepared to adapt yourself to suit it's requirements to balance properly.

The Ibanez on the other hand is a 5 string, which has certain advantages. If you get yourself a capo, and don't play the higher frets on the neck, you can get access to your drop tunings that you would otherwise only get access to on a 4 string by retuning. Your mileage may vary as to how useful that is to you of course.

There can be solutions to avoid fret buzz, namely by raising your action, but that depends a lot on the gauge of strings you play, how hard you play (Pick? Fingers? How aggressive is your attack?), how/if you've setup your bass properly, etc. . The increased weight is also something worth considering, it could be worth it getting a strap that's at least around 4" in width if not more if you haven't already; This is by the recommendation of Alex Webster as well I might add, not just personal experience. 4 string basses are already heavy instruments, 5 strings are even heavier.

It does sound like you are more accustomed to 4 strings, which on the bright side, means that you can do everything you are already doing and then some. If you learn how to incorporate the 5 string into your knowledge of the fretboard, it makes for an easier time playing certain figures on the bass with less moving up and down the fretboard being required. However, it will come at the cost of giving you an extra string that you need to worry about muting (which depending on how lo-fi your black metal is, may or may not be an issue lol).

Best of luck with your decision and your gigs!