> ... but nothing comes after death
Rebirth being untrue does not imply nothing comes after death. Even Buddha knew that he did not know what came after death. His teachings were about how to live, not about how to win at death.
> But Buddhism has always proscribed suicide ...
Not true. Himsa is proscribed, because it causes harm, and self-harm is still technically himsa, so a simplistic reduction might interpret suicide as proscribed. But such a reduction would also ignore all nuance.
Suicide causes harm. Not just to you, but to your loved ones, and possibly even strangers. Even if we restrict ourselves to an entirely aelfish point of view — where we ignore any auffering we may cause to others — unless you know, for a fact, that there is nothing after death, you cannot believe that you will not have suffering after death.