Yeah, Gero made calculated predictions on how powerful Goku would become based on spying on his various battles all the way up until he left for Namek.
Of course, history was changed due to alternate timeline Cell coming back to their past coupled with Goku not dying when he would have is what resulted in 17 and 18 (and possibly 16) being far stronger than they "originally" are in future Trunks' timeline, where at least a normal, non-ascended SSj could still put up a fight and, one-on-one at least, even had a slight chance of possibly winning like we saw with Gohan vs. 17 up until 18 barged in. With Goku's original death, Gero would've had no more purpose in continuing to make 17 and 18 any stronger as their target no longer existed, so probably just stopped right there as soon as word of Goku's death reached him and turned to the usual world domination stuff as the Red Ribbon Army wanted to do. 17 and 18 also being so much stronger is also most likely why Cell became so powerful after absorbing them, which makes me wonder how much weaker Cell would have been in the future even if he managed to absorb 17 and 18 (given there were far fewer humans for him to absorb compared to the past), and if Ascended Trunks could've possibly been able to defeat him even in Perfect form due to being that much weaker as a result of all that. Of course, Trunks wouldn't risk it, but still, it's interesting to think about.
Though, I still do question on why Gero made himself into an energy absorption type of android instead of simply making himself an uber-powerful "normal" model like 17 and 18 were.
Pretty much this...
This whole time travel/ paradoxes thing confuses me but you managed to put in a way that I could easily understand 
I think Gero made himself into an energetic absorption type of android mainly because
1) his body was far too old to handle the operation 18 and 17 handled
or
2)He needed the energy absorption power to research things in a more efficient manner
Persona 5 hype 
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