I think the next volume will be the final one. I love Rom. It was one of my favorites as a kid. I long ago gave up on any hope of ever seeing it collected as Marvel’s licensed stuff from back then rarely is collected. These Omni’s are a dream come true.
Rom's a nostalgic trip for me. I enjoyed the single issues back in the 80's, although not in sequence. It was a time when comic book distribution wasn't great and comic book shops didn't exist. Whether or not you want to pick up the 3rd volume is entirely up to you. How did you find the stories? Did you enjoy it enough to pique your curiosity to continue reading the subsequent stories? If so, then by all means get it. If not, then there's no point spending money on an omnibus. Even at discounted prices, they still cost a pretty penny.
I'm English and I had the Rom action figure, it was part of the Action Man range made by Palitoy in the UK. Also managed to pick up most of the comics from my local newsagent as they came out, not bad considering the patchy distribution that we got in the UK before comic shops were a thing!
Like you, I also didn't collect ROM comics or the toys. I think I do have a couple of issues, though. Likely some of the guest appearances. Regarding the omnibus size, however, I prefer these slimmer volumes as they are easier to handle and read. Monsters like the Fourth World books are just too unwieldy. These ROM books, Master of Kung-Fu, Micronauts, etc are far better for that, IMO.