Ok basics:)
Flush toilet with bucket of water, does it always fush OK?
If it does NOT then pull toilet and make it into a big project.....
If using a bucket flushes properly the toliet asnd drain lines are fine:)
the problem is the internal passages inside the toilet that send water around the bowl rim.the water comes from the small line that goes to the dip tube in the tank
these passages and exit holes under the rim can get sediment buildup over time, it slows water flow,.
flush toilet and watch water in bowl, does it just swirl around?
if so acid is your friend for 5 bucks and a few minutes time its easily fixable. its safe effective easy and worth trying before you pull toilet, which can have downsides.
like ooops the flange cracked, ooops the water line in the wall has developed a leak, the wife says as long as your pulling the toilet lets remodel the bathroom, etc etc etc,..,
try the bucket flush and report back here with the results