In the past I have done plenty of multi day walks. I have used the following kit to keep me dry.

Gore Tex Pro shell jacket
Gore Tex gloves
Paramo trousers
Event Gaiters
Gore Tex boots

This combination has served me very well.

However I want to do some bikepacking and I don’t know what to do about clothing this time of the year I ride with.

Normal SPD shoes
Walking socks
Leggings
Regular sports shorts
T shirt
Rain coat

Now as I’m normally out for no longer than 8 hours I dont care that I end every ride soaked from waist down from either puddles or rain. However this would be no good for bikepacking.

Any suggestions?

Neoprene socks and gloves. They work better when wet.

Quality set of Mudguards

I like to make sure I have materials that will keep me warm even when wet so I second using some neoprene such as in overshoes and gloves, but also swear by using merino such as baselayers and even old jumpers.
Are you camping if it is multi day? in which case I’d advise a down or primaloft you can put on when you are not working.
What kind of bikepacking are you doing in January? I salute you.

Stick to man made fabrics for over-nighters, especially at this time of year. Get merino wet and it’ll stay wet.