It looks like you went with 1/4" steel. Much better choice than the 3/16" that an unsaid company uses which is honestly too thin. I have a 3x4 ft and 2x4 ft table from the unsaid company. The ribs from them would arrive somewhat in a mild banana shape which would cause the table top to either dish or dome. I spent a long time on the tables to get each part as straight as can be with a shop press. When I would clamp them, the tables would be within about 5/1000" or less over the length of the table, but once welding started, even tack welding, the tops did distort. Going opposite corners, leaving time for cooling, just didn't matter, they would go beyond their claimed 15/1000" tolerance. Oh well. My 3x4 is actually pretty good and has let me make plenty projects square over the last 2-3 years. My 2x4 top humped a bit so I use it to hold tools more than for an accurate welding station.
I like your idea. I would caution you that the unsaid company has a patent on the "t a b and s l o t" connection for a welding table assembly.
I do like their welding tables because they are essentially mft type tables. I've made 5/8" steel dogs, layout for squaring corner welds. It does work great. I even use the Festool screw clamps on the welding table.
Don't know where you are located. I would interested in either a 2x4 ft or 3x4 ft version in your design with 5/8" dog holes. A top only, a base can always be self created.