Welding with Smooth Internal Finish

does the thickness allow for a "backstop"... a small strip of SS316 tacked to the inside of the weld, overlapping the joint would prevent weld splatter.

Google "electropolishing".

This forum has no way of assessing the capabilities of the undefined <...we...>.

You purge the pipeline with an inert gas like argon or a mix gas like a trimix such as 90% Argon, 7.5% Helium, 2.5% Carbon Dioxide.

now not everyone can weld sanitary piping, you need to get a competent welder to do that.

I like to add, if done correctly, the inside finish of the seam will mirror the out side weld bead. if the material wall is thin where a single pass is enough.