Im going to a 1x is there a freewheel with more than 34 t on the large gear/cog?

The largest freewheel I found was a 10 speed with 14-36 teeth, or Shimano's "megarange" ones that generally go 14-32 or 14-34.

The rear mech will probably have a model on it. Searching for "brand model spec" will often give you a manufacturer's specifications page. To use an arbitrary example, a Shimano Tourney RD-TY300-SGS google returns https://bike.shimano.com/en-NZ/product/component/tourney/RD-TY300-SGS.html

That page says in part:

Aside - your rear derailleur doesn't care if you have a freewheel or a cassette on a freehub. But it has to match the rear wheel's fitment.

So find your model, search for the specs, and see what its capable of. You may choose to push the published stats by a little, and probably get away with it. Something rated 34T will probably be fine at 36T, but 40 would be pushing it. 50 is right out.

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Instead of going big at the back, you can also choose a smaller chainring at the front, with the natural consequence of not having a high-enough gear for a big-tailwind or a downhill.

Another option is to have two 1-by chainrings, and use a mechanical system to change from one 1by to the other 1by. I've heard its possible to have three 1by chainrings on the same bike, potentially giving a sub-1:1 gear ratio at the lowest gearings.

^{Seriously though, there's no shame in a double or even a triple chainring if you ride a lot of hills, or even just the occasional steep one.}