As a certified professional welder, this is how I view you dilemma(s)
With a single vee bevel, I can run a good but joint @ about 90 amps, in all positions, 7018-1/8" rod. This is using a 1/8" 6010 rod a lower amps (don't really remember the setting) root pass. This is on 3/8" ANSI A- 500 (A-36) Plate, with and w/o a backup plate.Leave a 1/16" land on the bevel and also use a 1/8" root gap. What I am trying to say is that I do not think that your machine is to small.
With respect to the slag inclusion, it sounds like you angle of attack is incorrect. Use a lead angle of about 10 degrees while holding the stinger and rod at about 45 degrees with respect to the work. This is what I do. Also make sure that your arc length is correct. With a 1/8" rod your arc length should be about 1/8" ( if I remember correctly)
Take all of this information with a grain of salt, I have not used SMAW (aka stick) for about 5 years.
I had a hard time with stick in school, but I felt a great sense of accomplishment once I got it. My suggestion like other would be to stick with it. You can do a great many things with stick, and due to the low amount of consumables required to stick weld as compared with GMAW (MIG). I have a MIG machine I like it it was expensive and I will take me a lot of projects to equalize my ROI (rate of return). All told I spent $1,000 on the machine and consumables. Another $500 on a torch and bottles.
