I remodeled a bathroom about 6.5 years ago. The toilet I had there before never clogged. The waste line for the toilet did not change during the remodel, except that a fitting was added to the toilet waste so a shower could be added. The new toilet is a Toto MS854114S 1.6 gal.

The toilet only clogs when a certain member of the household deposits solid waste in there. This person was away for 2 years and the toilet NEVER clogged during that time, and it does get used.

QUESTION is:
1) Could this be an issue with the waste plumbing, considering that it's mostly NOT a problem?
2) I'm thinking this toilet simply cannot handle especially hard solid waste and maybe that's the problem.
3) Is this model toilet known to not handle a large amount of solid waste very well?
4) Would changing to a different toilet that has a very large clearing capacity be a solution?

Obviously the member with the large stool deposits needs to eat more ruffage. Improve his or her diet ant the problem will go away.

fill a 5 gallon bucket and dump it into the toilet as fast as it will allow you to.
you will know it there is a restriction in the piping below.

that will NOT test for a 1.6 gallon flush.

I tried that, and 5 gallons of water went right down with no problem.

So... does that mean the problem is NOT in the waste plumbing?
If so, then will swapping the toilet to one that can handle more solid waste at a time solve this problem?
And if so, which toilet(s) would you recommend?

That's not a great flushing toilet. The MaP testing results were 600 grams.

At some point you have to draw a line. I have one grandson that could clog a sewer main. Ask the person that always has the problem to flush mid way through the finishing process. It is probably the volume of paper being used that is creating the problem.

Just installed a Toto Entrada 1.28 in the powder room and so far no issues. Very powerful flush. Your best bet I think would be to replace the fixture with one that handles solids better.

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Not a plumber by trade but a fierce DIYer

Low flush toilets are always going to be a problem particularly if the waste lines are flat or have a belly in them. You will need to camera the lines to determine if that is your problem. Absent problems with the lines (flow), low flush toilets can work perfectly. I have Kohler K-4634 toilets in my home and they are near flawless. The Comfort height bowl is a plus especially as you age.

On the advice of members of this forum, I installed a Toto low volume toilet about 4 years ago. That toilet needs a second flush less often than any other toilet we own.