This is not really an answer. Just a too long comment to post in as a comment.
EDIT
I have quickly had another look now. I think you could just (globally or in your preferred evil-states) rebind the next-buffer and previous-buffer functions to your preferred keys. You can use e.g. C-, and C-.. Although they are bound to flyspell and evil function by default, I never use those. So you can unbind them and bind the buffer switch function by adding the following lines to the user-config section of you .spacemacs file:
(define-key flyspell-mode-map (kbd "C-,") nil) (define-key flyspell-mode-map (kbd "C-.") nil) (define-key evil-normal-state-map (kbd "C-.") nil) (define-key global-map (kbd "C-,") 'previous-buffer) (define-key global-map (kbd "C-.") 'next-buffer)
Then optionally you could activate also tab-line-mode.
END EDIT
Personally I think SPC b n is inferior to using e.g. C-tab like in most standard applications. Emacs 27 provides tab-bar-mode so that you can use C-tab for switching between window configurations. Also there is a tab-line-mode which lets you switch between buffers. I think you should play with these modes and rebind tab-line-mode's previous-buffer and next-buffer functions (e.g to C-, and C-. which correspond to the < and > keys without pressing shift). There is an unfinished centaur-tabs layer that you can try and install and from which you might get some inspiration. I stopped working on the layer as I thought I should start from the basic tab-bar-mode and tab-line-mode functions. Also I think even the tabs are unnecessary, we could just cycle over buffers while showing them in the minibuffer (instead of listing the buffers in tabs). But a package still has to be developed for that (I think).