Water resistant only goes so far but full on rain gear is too stifling and you end up getting sweat wet....so it is better than nothing. I have found that spending alot on "breathable" gear doesn't work for me either. I just use lightweight wind and water resistant, and I might add cheap as in got off eBay, jackets. I have an orange one for hunting season, just over, and a few others for all else. Make sure you get it loose fit for layering.
For pants I wear these:
https://www.amazon.com/dp/B0777BL8KD?psc=1&ref=ppx_yo2ov_dt_b_product_details They are insulated just enough, breathe ok and haven't ever soaked them through although they do get damp. I live out on the coast so I find myself wearing them most of the time on the bike and off actually as they are very comfortable.
To keep my head warm I wear a hoodie appropriate to the temp under and put my helmet on over the hood. I tried a hat for awhile but when overheating it was a pain to stash it if removed and a hood just flops off.
I use insulated gloves, Amazon/eBay, that have little flaps to allow you to use your finger tips, handy for phone access. Look for ones with neoprene content. I wear a pair of Grundens short rubber boots that work well with flat pedals and have enough grip when off the bike. Too warm here in the OR for insulated types although I have lived where they are necessary in the past.
For really bad day outings, not store runs, I purchased a set of water proof coveralls recently that I have yet to try out. There are some some companies in England that make some that look good but are $$$$ but my eBay ones were cheap. I have always felt that for MTB riding on a crappy day it has possibilities. I'm sure there will be a time for a trial in the near future.
A good set of fenders goes a long way here in the OR also.