Can this line be improved?

I live in a 2 story townhouse where the guest bath toilet is the lowest open point in the system. About every two years, the line from it plugs up and it overflows, usually while doing laundry, and nearly ruins nearby hardwood flooring. The pictures follow the line from below the toilet to where it is joined by washing machine drain, then to where it is joined by kitchen drain, then down to main. The wye where the kitchen drain comes in is where it plugged this time. The original ci pipes were installed in '64, and the new ones in '98.
They are 3". The plumber who cleared the line for me said there is not much I can do to prevent this, except maybe using one of those hose jets in the cleanout (upper left pic #4) once a year. I'm wondering if replacing all of the horizontal parts of this line with ABS would be a good idea, perhaps going to 4"? Anyone have any suggestions?

drain

From the pictures we cannot see any physical reason why it happens, but then we also cannot see the slope of the various drains. A minor point but I would not have connected a branch line using an elbow and a combo with a rubber cap on the end as shown in one picture.

One negative point about ABS and PVC drains is they are noisy. Every time you run water through them, you will hear it. I would suspect the problem lies in the slope rather than the drain line material. You may just have to live with an annual jet cleaning. I don't have much faith in those DIY hose attachments.

Another thing to consider is what you are putting down the drains. You might consider a lint filter for the washer output, and try to limit the amount of solids going down the kitchen drain (do you use a garbage disposal?).

There are also some safe (not harsh chemicals) drain cleaning liquids. Don't know how likely it is that they might help in your situation (depending on what is causing your blockages).

Thanks for the replies. I measured the slope at around 1 1/2" in four feet all along the pipes. We have no children, so no unidentified objects get flushed, nor do we put anything but tp down the toilet. Yes, we have and use a disposal, it's a brand new insinkerator 3/4 hp that's supposed to only expell fine stuff. I think the clog happens in the bathroom branch of the wye anyway. Is there any chemical or enzyme, whatever, that you have found to actually help keep drain lines clear without harming the pipes?