Maybe ask @gtaus for advice?
He has cheap solutions for many things.
Like using free pellets for almost everything.

Thanks for the shout out. Well, I guess I fit the description that if I can solve a problem with free pallets, that is what I will try.
I've been thinking again. Yeah I know it's dangerous.
I built the frame the large Solway coop sits on out of wood and OSB board.
The board is already suffering from the elements and rat chewing.
Marine ply would help and the coops I built in Catalonia with marine ply fared better in all respects compared to ordinary ply. But, have you seen the price of proper marine ply these days!
This summer I had to replace some large sections of my coop floor. The OSB, under a linoleum sheet, had rotted out and actually broken through in a few spaces. It was soft and spongy in other spaces. My OSB lasted about 5 years, so I'm not complaining. But I think marine plywood would have lasted much longer. Like you, at the time, I was not willing to pay the high price for marine plywood for my chickens which I did not expect to have more than a couple of years.
I suspect my OSB flooring rotted out from the bottom up. Someone mentioned that if I simply painted both sides of the OSB before I put it down as floorboard, it would have lasted much longer. I think that is somewhat true, however, even my exterior OSB which is painted is starting to rot out after 5 years.
In any case, my solution for new floorboards this summer was to cut up a bunch of pallet wood planks and lay them on top of the old floor. My new pallet wood floor is much stronger than the previous OSB floorboard. I use dry deep bedding in my coop, so all the pallet wood flooring will be covered. That means I did not worry about some planks being thicker or thinner than other boards. In my case, I just cut them all to 32 inches long and tacked them down.
I did not bother to buy any linoleum this time. Since my pallet wood floor is free, I'll just replace any boards that eventually rot out. I clean out my coop bedding twice a year, so if I notice a bad board, I'll just pry it out and replace it with another. Since everything is cut to a uniform 32 inches, I'll just have to match up a board with the same width or rip it to the correct width. Either way, not much of a problem.
I built my chicken coop a few years before I got into using free pallet wood for my projects. My OSB lasted about as long as I expected, but it was not the best choice for a long-term project. Having said that, I truly thought I would have chickens for only a couple of years. But here I am at 5+ years with that coop and still enjoying a backyard flock.
Here is a picture of the first half of my chicken coop floor being replaced with pallet wood planks...
You can see I just laid the planks right on top of the old OSB flooring. I laid the planks in the direction that I sweep out the coop, in this picture, it would be from top to bottom. Since the planks are not all uniform in thickness, I thought sweeping out the coop litter would be easier if I had the boards laid out in the top to bottom orientation.
It's not a perfectly level floor, but since I will have 8-12 inches of dry deep bedding litter on the floor by the end of the winter, the difference in board thickness will never be noticed. The pallet wood floor on top of the old OSB is much stronger than the OSB floor was by itself.
Marine grade plywood starts at about $120.00 per sheet here and goes up from there. It would have cost me $360.00, or more, to redo my chicken coop floor with marine plywood. The pallet wood was free, and it's easy to replace any pallet boards that eventually rot.

I don't know about you, but $360.00 in my chicken budget will buy a lot of feed for a number of years. I go through about $10.00 of feed per month. I'm OK with a free pallet wood floor to save money.