Not sanitary if welded from outside, unless you get full penetration.
Consider the weld joint geometry - if it leaves little crevices on the side that is supposed to be clean, and able to be cleaned, it is not sanitary, because those little crevices will hold crap and not get clean (and they will also hold traces of any chlorinated cleaner and promote pinhole corrosion).
But I'm just an amateur. A beer-brewing amateur, so I do have some grasp of what's needed, and I believe we did discuss some of this WRT milk lines when I was learning to weld in the ag-eng department.
And if a weld breaks in a pin-hole - or small hole - it might be inside the tank. Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder
The best solution is to get a ball-bearing ball the size of the inside diameter of your clover (tri-clamp) fitting. Most tank fittings I have welded had a 1-1/4" inside diameter. Drill a hole about 3/4" diameter in your tank.
Weld the ball to a threaded shaft and make a simple jig to pull the ball through the hole with lots of wax or bearing grease as a lube. You will want to have a larger piece of pipe on the outside of the tank to draw against.
What you end up with is a raised shoulder coming out of your tank. Makes it much easier to weld the fitting on. Many commercial tanks are done this way now.
When I have had to replace fittings on tanks I have used a hole saw to remove the old fitting and the weld around it. I then weld a stainless steel donut to the base of the tank fitting. The donut is sized to just fit into the new hole. I weld the donut onto the fitting on the weld table.
Then I weld the donut to the tank inside and out.
It is a bit easier than trying to make a smooth transition inside the tank.
Of your three choices, I would go for the hole in the tank the same size as the outside of you fitting base. Slide the fitting into the tank so 1/16" protrudes into the tank. Tack it on the outside, set up your purge ring around the outside, then weld it on the inside using the edge of the tube as the filler metal. Then back purge the inside and weld the outside. A little cleanup with a die grinder Then passivate inside and out.
The ball bearing trick is so slick it is really worth building the jig.