the OPs cranky toilet assuming it worked well when new may be from
sediment, hard water buildup in the toilets interior passages. when
they get clogged toilets dont flush solids as well.water kinda swirls
around

plunge toilet, try to get all water out of bowl, water can remain on
and tank, the top half full of water.

open window, wear safety glasses and use a funnel to pour muriatic
acid in the dip tube,

about a pint, avoid splashing. wait 10 or 15 minutes, then repeat.

when done flush 10 times to dilute any acid left in lines etc.

muriatic acid sold in stores is pretty mild, I put some on my skin
just a little warm, ended instantly when rinsed off its a mild acid.

After doing this for myself and friends and posting it here others
report how well it works.

muriatic is mild acid and not hazardous if used with care