It’s Federally legal, unless otherwise restricted by state law, to manufacture a semi-auto firearm. Texas has no laws restricting manufacture of a firearm other than you can’t make an improvised firearm out of non-firearm parts.
Federal laws *do* say you can’t make the firearm for sale or distribution, but you can sell it, give it away, etc. later on if the thing was originally intended only for personal use. If you transfer it, you have to put a serial number on it even though you need not put one on at time of manufacture.
No, marking requiements, ie. Serial numbers, only apply to licensed manufacturers, at least with regards to Title I guns.