Zamasu never became the universe, that is just not true, he was going to, but he was not by the time he was erased. Gowasu even claimed he wanted to became the universe not that he already was, and as seen when Zeno destroyed him, he wasn't the universe yet.
Why are the stars far away not merged with him if he already was the universe? how come was he depicted being just one planet and some space around him when they could at least make him cover the whole solar system?
Ok, cool, that still doesn't remove the fact that Infinite Zamasu merged with the very fabric of the Universe and became justice and order itself, which is a mastodontic achievement. And he was transcending time and space too.
Oh, and Gowasu stated in ep. 65 that Fused Zamasu had endless power because he was the fusion of the most powerful Goku Black and the immortal Future Zamasu, so when the two became one, their power didn't just merge, it expanded to no end. But, hey, surprise! It turns out that this guy Jiren is beyond anything they've ever seen before, even Zamasu, the guy who had endless power and was beyond the Gods' comprehension. Oh, I can't wait for the new Saiyan villain to be 'beyond anything they've ever seen before', including Jiren.
And three depleted saiyans managed to block Zamasu's attack and save Bulma, Shin, Gowasu and the ship
No, they didn't block that attack, they were overpowered and knocked unconcious.
At his core Zamasu is good like Shin, though I guess you could say he was so fastidious that it backfired. But you know, for this "Future Trunks Arc" you had to depict Zamasu and Trunks' inner conflict, right? If this was back when I was drawing the manga myself then I doubt if I could have done it. I mean, I'm not very good at depicting the characters' psychology on the page. So this all came together because now I only have to think up the story. [...] On my own, I doubt I would have been able to express Zamasu's fall to the dark side.
Akira Toriyama, DBS vol.4 joint interview with Toyotaro