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Just pulled my front bumper and you'll need a 18mm and a 15mm socket.
Agree with you, I just pulled mine off to install the front receiver hitch. But the OP's question seems like he is referring to some other bolt when he described a "spline looking" bolt. Perhaps he assumes the wrong bolt and is seeing a Torx type fastener.?
Pulled mine off my 2020 withstandard metric six points. Cj
Do you have to pull off all the plastic trim inside the bumper to get the bolt cover trim pieces off? I need to remove mine eventually to install my plow mount so I've been looking at it, but I haven't gotten too involved yet. The hidden bolts on the grill were tricky to find.
The only plastic removal on 2017-2019 are the plastic covers around the tow hooks which can be removed by hand. The four 18mm bolts around the tow hooks are anchored to a plate behind them so back them out equally. The plates are vertical and if you entirely remove one bolt the the plate, it will spin and you'll be stuck until you put the other one back in. The two 15mm bolts are accessed from behind the bumper in front of the wheel well.
When replacing the front bumper I used two floor jacks with short 4x4" blocks to help align the bumper.
The only plastic removal on 2017-2019 are the plastic covers around the tow hooks which can be removed by hand. The four 18mm bolts around the tow hooks are anchored to a plate behind them so back them out equally. The plates are vertical and if you entirely remove one bolt the the plate, it will spin and you'll be stuck until you put the other one back in. The two 15mm bolts are accessed from behind the bumper in front of the wheel well.
When replacing the front bumper I used two floor jacks with short 4x4" blocks to help align the bumper.