Typically, after welding, the properties near the weld are those of 6061-0, a loss of strength of around 80%. The material can be re-heat-treated to restore -T4 or -T6 temper for the whole piece. After welding the material can naturally age and restore some of its strength as well.
Make sure you're cleaning the weld area with a stainless steel wire brush -- you can get small toothbrush sized versions - and be sure to clean all the areas including the ends.
4043 fill rod is fine. You failed to mention what tungsten you're using... and it looks to me as though you may be getting some tungsten in your weld. Use Zirconiated tungsten not pure or thoriated.
Heat is typically 1 amp per .001 thickness... so 1/8" would be 125 amps. I would not bevel this joint. It's not needed and you're replacing the 6061 strength with the weaker 4043 fill rod by beveling like this.
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