One of the beauties of using epoxy as a tip reinforcement is that it can be easily repaired/renewed. It's meant to be sacrificial and to wear as the paddle is used.

There's no real need to add thinner to the oil, as there's very little to be gained by doing so. The amount of penetration anywhere other than on end grain is tiny and thinner won't do much to improve that.

A file is not going to put nearly fine enough of an edge on a plane iron; it needs to be honed properly to a consistent angle. It should be mirror polished at the edge and capable of shaving hair with ease. If you do a Google search on plane sharpening, you'll find more videos and articles than you could possibly watch. You don't need to spend a fortune on expensive sharpening equipment, but you do need to make an investment is some sharpening system. I've tried several and prefer water stones, but one of the sandpaper-based systems will suffice and save some money. I strongly recommend getting a honing guide of some sort as well. An inexpensive side-clamping guide will do the job.

Inexpensive planes generally require a fair amount of tuning to get them to function properly (sole flattening, removing excess paint and burrs, etc.) Again, there is a huge amount of material on the web about tuning a block plane. It can take some time, but it's neither difficult nor expensive. I tuned a bunch of inexpensive planes for a class and they work fine for soft woods like cedar.

A properly sharpened and tuned plane will be capable of producing shavings so thin that you can see through them. It's a joy to use and produces an incredibly smooth surface. The difference is absolutely night and day and it can dramatically improve the quality of your projects.