The Overton window describes movement on a political axis by views held palatable to the public but does not reject the concept of objective definitions of "right" or "left."

For example, the Overton Window would imply that there were right, left, and center Nazi party members. And there are, in the frame of the Fucking Nazis, but that doesn't make a "left" Nazi an actual socialist.

Saying "Macron isn't center" can be taken as an objective assessment of his politics. And it's correct. He is not a centrist. Who don't actually fucking exist anyways, but whatever.

So, basically, OP is saying you're playing the fascists' game when you let them define politics by relativism instead of objectivity. Bowing to the terminology of the Overton eventually leads to arguing about which "centrist Nazi" you find most palatable.

As I see it, the biggest frustration from the left is that he makes right-wing changes.
The right pretends to be frustrated by his policies because they need to disagree if they want to have a reason to exist - and they've been leaning towards the far right for years to find a place on the political spectrum because Macron took their place.

A few examples, I hope I'll be able to explain correctly.

So in France we got something called the "prudhommes" which is a board that specializes in dealing with employee vs company legal matters. Usually, when you get fired and you think they didn't do it right, you go to the prudhommes and you can sue them for money.
The prudhommes are important for the workers because they can charge bug fines to the companies, and once one worker has won their case, it becomes easier for their colleagues to do so if they were wronged in the same way.
It's a powerful tool that kind of forces companies to follow the "code du travail" aka workers' rights.
When he changed (read: degraded) lots of stuff about workers' rights just after his first election in 2017, Macron introduced an important change: there is now a maximum amount that the prudhommes can force a company to pay. This changes the balance of power because now companies can now how much breaking the law will cost them and decide that it's the cost of doing business. Worst thing is, this amount can be changed by the government at any time without requiring a vote from the national assembly - this is read by many commenters as the start if the prudhommes' dismantlement.

In France, we've had (I believe since the eighties) a kind of universal basic income. It's far from a livable income (it's around 600€/month currently) but it's still more than nothing. You need to be french (we wouldn't want to share nice thingd with refugees, right?) and jump through a few hoops to claim it, but it exists. Until Macron, the only condition you had to fulfill was having no other income.
This is used a lot by people who are unable to work but can't apply for unemployment or sick pay mechanisms. It's always been the last safety: when everything else fails, at least you get something.
Since Macron, you now have to be actively looking for a job to qualify for it. Which means that if your unemployment office thinks you're not looking hard enough, you can get excluded from this income and have absolutely nothing (which then leads to homelessness and a whole life of fun). And of course over the past 7 years of Macron, the unemployment offices are under an ever increasing pressure to find people who are not deserving enough.

In terms of image, Macron took lessons from Trump: truth and facts don't matter and you don't even have to be coherent. Time and time again, he and his governement have lied, contradicted themselves... They are of course not the first to do it, but they are the first to not give a shit. Before Macron, they tried not to get caught lying, and when they got caught they tried to get out of it "that's not what I meant". Sometimes they even apologized. Macron and his government have had a different strategy: we don't even care.
When Macron visited the french carribean, he said in a public discourse "the chlordecone [a nasty pesticid wildly used by french companies in the carribean until y2k] is not cancerigen". Right after the discourse, scientists and physicians were like "well yes it is, it's been known for 30 years". Macron didn't even bother answering himself, but his services blantly told the press "he never said it was not cancerigen" even though he said it a few hours earlier in front of all tvs and radios. This is just one example of something that now happens weekly.

These changes might seem small, not structural. But they change fundamental things in the balances of power, the way we treat people and the way we do politics. Especially since there are lots and lots of them in a lot of areas, when you take them together, french legislative and checks/balances landscapes have changed a lot in 7 years, much more so than they had since the nineties (Idk before that).

Macron is from the bourgeoisie, is fighting the class war and has been waging and winning more battles than his predecessors by a lot. He is not interested in the status quo at all.

I hope this makes sense and answers your question ;)