waht tyre is rear using now? How old is it?
does the rear tyre slide in the wet when u turn?

in low grip situations (rain, gravel, sand) usually we try to brake upright using the front, rear brake will make rear tyre more easily lock

I can recommend Corsa platinum series (R26, Cross etc), Battlax BT39.

My experience tells me, is the rubber compound that determines wet grip, not the tread pattern. Tread pattern determines likeliness of aquaplaning, which is not wet grip.

the two models, are the extreme cheap and expensive tyre I have tried both wet grip are good
no matter how good, can still lock so bear this in mind

as long as once u release rear brake, the tyre grip back, is good liao - u haven't seen tyre that continue to slide even after brake release, this one really really bad

existing tyre can try use slightly lower pressure to improve grip

u try shop around and see... if not buy from shopee and bring to shop pasang bagi duit upah

try to use a proper size tire.
u can try corsa sport rain tire or corsa cross S..
also try to control your acceleration and braking when the road condition is wet

which is? 70/90 and 80/90?

the tyre how?
I use same size as you, but they look bigger like 80/90 and 90/80... Corsa all like that, especially the Cross is even bigger

compare the sizes side by side, should be able to tell how much bigger they are
hope they are good grip enough for you