They weren't too complicated at all. In fact, what we see in the books on mythology now is the compendium version. At the time, different parts of the mythology were stressed in certain times and places, while others were ignored or relegated to the background. Complexity wasn't the issue.
The issue is that they were too tolerant. In any competition between two religions, the least tolerant one generally wins. Be exclusivist. Be very clear about the fact thay you and you alone are offering the Truth, and all others are heathens and liars who consort with demons and filth. Don't welcome other religions into your society if they are exclusivist. Toleration only works if the others are willing to syncretise and/or co-exist, too. If there are ten religions, and nine of them say that there are many gods, but the ones they worship are the most worthy etc. ...and one says only their God is real and all the others are demons from hell... then I can tell you which one is going to be successful in the long term. (Provided that all aim to convert: exclusive groups that are very closed, such as Judaism, limit their own "success" -- and by design.)
Exclusivity adds to the attraction of a religion. That may seem counter-intuitive. It might be more sensible to think that being the tolerant one makes you less divisive, broadens your appeal... No. It only waters you down until you dissipate. Religion needs fervour. Better to have a hundred really fervent believers than a thousand wishy-washy types who will just adopt new gods when that looks like the better deal.
Here's another counter-intuitive point: a religion needs to demand things from its adherents. If it tells you that everything is okay, that's comforting. It may seem reasonable that people want comfort from religion. Wrong. They want a religion that feels real, and soft stuff doesn't feel like that. Christianity has a God who died for your sins, and you cannot possibly hope to match that sacrifice. You can only aspire to do your best to measure up as well as you can. Being torn to shreds by lions because you refuse to denounce your God is literally the best possible ending you can hope for: martyrdom. The Romans were perplexed at this, because they didn't understand the sentiment. They didn't get why Christians didn't just keep worshipping their own God, but also pay respect to the deified Emperor. The idea of being so attached to one God that all other Gods -- and all authority of the State -- could and should be cast aside in favour of obeying this God... it was alien.
Finally, there's active proselytising. The Religio Romana cared very little to do this. It syncretised. It absorbed others into itself, and those others were willing to be absorbed. That, or (like the Jews) they were "only for the select few", and thus no competitors for the wider masses. A religion that attempts actively to convert others will have greater success than one that just kicks back and assumes itself to simply be the standard of the natural order of society. So you must proselytise (meaning you need a reason for that).
Find a way to make your religion intolerant, exclusivist, zealous, demanding and proselytising. That's the recipe for success. For the Religio Romana, this would almost by definition require an integral overhaul/rebirth (on the scale of Vedism into Hinduism). Contrary to popular belief, Julian the Apostate was actually one of the few people headed in the right direction to possibly achieve this.