Hi, Sho, and a belated welcome to the Forum.
Maybe I can help to add to what has been previously posted.
Cuben Fiber packs exist which are waterproof, do not absorb water, and are the lightest backpacks manufactured. They are expensive, but if you are going to do a lot of hiking and backpacking then the cost per mile is fairly small.
Many manufacturers will use a coated fabric for the pack bags. The fabric is waterproof. What is NOT waterproof are the seams on the bag produced when the bag is sewn together. BUT one can make the seams waterproof, which also makes the pack waterproof. One can purchase or make seam sealers which are liquids that after being applied to a seam, dry to a waterproof coating. Although originally made for waterproofing tent seams, Many have used these to great effect on our backpack bags. The amount, method, and instructions are the same whether for a tent or a bag.
Manufactured seam sealer. When I owned a Tarp Tent backpacking tent, I used their instructions for
a home made seam sealer
Modern synthetics used for most backpack bags and belt/harness construction will only absorb a small amount of water overall, and will dry out quickly. The shoulder strap pads and hip belt padding are made of non water absorbent material, like closed cell foams.
So the strategy of using a waterproof bag liner, like a kitchen compactor bag or a heavy duty utility plastic bag is a sound strategy. I do agree that pack covers can allow water to access a pack, depending on how heavy the rain is and how long the deluge lasts
Ponchos can be very effective if they are of a length suitable for going over a backpack. They are also multi-taskers in that they have a lot of uses besides rain wear. I've used the same poncho for thousands of miles thru hiking the Pacific Crest Trail, the Colorado Trail, and on my Caminos. If the weather looks or sounds like it might rain or drizzle, I keep the poncho in my side pocket where it is quickly reachable. Even without stopping, it is a simple matter to give it a couple of 'fluff-out' shakes and then slip it on over me and my pack.
The poncho, as you already know, keeps the bag and the straps and waist belt dry from water.
Wind and ponchos? There are techniques one uses to make that a non-issue. In fact, I look forward to breezes and winds if I'm wearing a poncho as that adds to the ability of air to move in order to keep perspiration and condensation at an even lower level.
As far as legs go, there are several issues at play. I like hiking only in shorts. If it is raining and I find the bit of rain that contacts my legs at mid calf to be uncomfortably chilled, then I will put on a thin base layer bottoms on under the shorts. Even wet, they keep my legs toasty. The other option I might use is an ultra lightweight rain kilt/skirt.
Wet legs themselves are no big deal to me. They dry off quickly and never get saturated. But, one can wear rain pants or a rain kilt/skirt if wet legs are somehow bothersome.
This is an issue whether one chooses a rain jacket approach, a poncho, or walking nekkid.
Concerns which are really anxieties about walking and hiking in rain tend to deflate to a low level 'yuck' as experience with wet weather walking increases. Experience leads to Knowledge leads to Confidence.
). Weight wise and ease of use “for me” I prefer rain jacket + rain kilt + backpack with rain cover and dry sacs vs the poncho
