Any reason why you don't want to pit a threshold there? That's kinda what they are for.

Get some molding for the transition between rooms? You could cover the poor cut with that

Check your lumberyards for sill plates and thresholds.

It's obviously too wide for a transition. The best way to solve this is to fill it. You'll never match the edges/colour so don't try.

Personally, I would install a marble threshold or a [tile mosaic] (https://atlastile.com/wp-content/uploads/2020/01/tile-accent-border.jpg). That would let you use grout to hide the imperfect edges on the floor and get the height right.

I'm a bit eccentric. I would get colorful tile and make a mosaic in the gap. It would add a little pop!

Use a straight edge to continue the poorly cut piece's edge through onto the adjacent piece. The best way to do this is to put down a layer of making tape on the floor, and then a layer of making tape on a ruler or level.

Use glue (Elmer's, wood glue, hot glue, whatever, just NOT super glue or epoxy) on the tape to stick the straight edge down in position. Be really slow and careful to make sure it runs straight from the bad cut to the doorjam. Let it dry.

Then use a brand new box cutter to run along that edge and cut through the flooring piece. You'll probably need to snap the blade off every four or five passes. Go slowly so you don't walk the cut. Once the board is cut you can work a painter's scraper under the masking tape and get it off the floor clean, no mess. Now you have two straight edges on the gap that aren't square, but they're straight.

Then you make a template of the area with cardboard, because again it's not going to be square. Just tape a few pieces together to get your edges and things right.

Get yourself a transition from the big box store, or some other board that's a contrasting color. Whatever it is, it needs to be thinner than the surrounding floor by at least 1/16". Use your cardboard template to cut down your new board. If it's wood, buy yourself a little hand plane and use that to dial in toward the size of your template.

Then you use a thin layer of titebond adhesive on the floor and stick your new board down. Be gentle with pushing it down so that you make sure it goes even with your existing floor all the way around. Fill in perimeter gaps with black or grey caulk.

Then try not to look too closely at it ever again.

Fill it in with fixall and then custom cut a piece of 1/4 inch thick oak trim to cover that area.

If it was me I would try to fit a piece in that gap and make it seamless. Using some filler compound or something to fill in any small gaps. Putting a threshold there would always feel like a band aid to me.

The flooring company might make a threshold/transition piece that will be of the same style and color. They probably have their catalog online.

I would use some of the store existing flooring and cut it to the right width (cutting the recess side, not the tongue edge), then fit it into your existing floor. The only issue then would be that the groove side(or what used to be that until it was cut off); will have no way of interlocking into the existing flooring, but I would nail it down if it was wood subfloor, or perhaps stick it down with pink grip or similar on that edge and also the edge of the floor meeting it. Yes, it is supposed to be a floating floor for expansion, but I don't think you will get any issues with it.

Failing that use a proper tile threshold to keep them together, might look nicer depending how good at DIY you are.

The row you fit won't be as wide as the other planks but it will at least easily fix this issue and when all the doors open, it will all match.

Quick note, if you are doing this, then make sure you knock all the planks together so there's no gaps(can put trainers on and kick them, or a tool if you can get to each bit) which can appear after some time. May as well use it to fix any other niggles with it .

If you don't have any spare planks, take one to your nearest hardware store and try and match their style and colour etc.

I have faced several of these types of transitions. If you have some of the original flooring, you could try to meticulously trim/fit a plank and shim it to the level of the floor then secure it down with construction adhesive, but I've found trying to make it "invisible" is difficult, and matching the gap, with its imperfections, is a meticulous process.
Next best (if you're good at matching staining and mid-level woodworking) is to take a plank and cut it into a threshold that fits relatively tight to that gap with some overhang onto the flooring, stain/varnish, then secure it in place. Finally, you can look for a store-bought transition that would do the same function as a custom built one - it'll likely not match as well, but it would certainly fill the gap.

Why not get a new threshold?

It creates a visual separation between the different divisions.

Maybe 2 transition strips with a narrow piece of the flooring between.

It depends on how picky you are. But it sounds like you probably will because it would bother you to much

You don't need to take up the flooring, you need a new threshold and not for lazy purposes: Such flooring expands an contracts due to changes in moisture and temperature by a few millimetres, so you have a threshold as a cover to let them move freely underneath.

You can cut a peice to fill the space and glue it to the sub floor.

definitely don't rip up the whole floor, that's a waste of resources.

Hi, I'm from Brazil and here we basically use to types: if it's narrow there are some like looks like the floor or "stones" like marble. And now I'm looking to the floor of my apartment and thinking that it needs to change too, help me! Idk why marble and stones are cheaper here so it's the most common. But maybe you can do with small tiles like smaller than subway titles?! You can do a art or just plain. If nothing else works I would fill the hole with something like any filler and throw a carpet over. Something that would go from room to room to give some identity. Good luck and show us the result!