I will be doing my uni body stiffening and adding sub frame connectors soon and just wanted to know the best metal thickness to use and about how much it will take

Most are a combination of 3/16 and 1/4".

There's no real subframe connectors because the rails run the length of the jeep, you just have to plate them. Most people run 1/8", 3/16" or 1/4". 1/4" is a little overkill imo and would start getting really heavy. I'm using 3/16" for the high stress areas and probably 1/8" for some other areas. Steering box area is most important to start with. Companies make nice kits for not too pricey. They usually bolt in though. Drill some Rosette holes and burn it in. Then add a steering box brace and tie the track bar bracket into the other side frame rail with a brace.

1\4" seems really thick
I was planning on 1/8" all the way down with some dom tube to connect the ends

This

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