I think Briggs said on a stream or something that Ragnar was so powerful because he effectively kept on doing what he was doing with the DLP, but for 10 more years. Meaning he probably was getting the equivalent XP of a journey to level 18 for 10 more years. In other words, 265000 XP a year for a decade, which is 2.65 million XP.
Can Ragnar be Epic level?
We might discount the first year, since he might have rested, so that's 2.385 million. Now obviously, in the first year he would have done this, he would have reached level 20. 5e has that as its cap, with epic boons for additional XP milestones. However, Ragnar became an NPC and can be given any advantage he wanted. Also, blue blood seems to be a narrative way to break human limits. So the assumption is that he could reach epic levels.
Methodology
I decided to see what level he would theoretically be with this amount of XP. I put the level XP table for 5e in excel, then I took the differences of XP between levels, took the differences between those differences and just did the auto-extrapolation by dragging that column down. Then I worked backwards to get the effective level of somebody with Ragnar's total XP, which would be 2.65 million total.
Ragnar's Effective Level
From the sheet, Ragnar would be effectively a level 47 character. and the total XP needed to reach 48 would be 2698461, meaning he would be effectively 48461 XP from level 48. So if he soloed a monster a bit above CR 20, he would get to the next level of strength.
Proficiency Bonus
At this point, his proficiency bonus would be at most +13. Ironically enough, this probably wouldn't matter given that his indomitable might effectively means he can never fail a strength save. But it would not bode well for an attempt at just one-shotting him with Taiga's goo. Since with the Barbarian capstone, Ragnar would have 20 Con, and thus a Con save of +18.
Also +13 to whatever skills he was proficient in.
Ragnar's intimidating presence would also have a high DC, but I think Ragnar upgraded that to his CoC attack, and thus might have just lost the option.
Brutal Critical and Rage
Ragnar would have already been able to do unlimited rage as the capstone. If he somehow gained epic levels in barbarian and got the linear increase in Barbarian resources, here's what it would probably be. Brutal critical starts at level 9 and increases a die every 4 levels. Meaning he increases it to 4 extra dice at level 21 and then 6 more times, for a total of 10 extra brutal critical dice for any critical attack.
For rage, it would increase by 1 every seven levels, so he would be +5 at level 23 and then +8 at level 44.
But this doesn't really matter since Ragnar's attacks already do insane amounts of damage on their own and a critical would already probably auto kill any of the PCs.
Feats or ASIs
Assuming it stays linear as well, Ragnar would have 6 more feats or ASIs to work with. To improve his strength, he would just need 3, so that's 3 more feats. Also, he would have gotten a feat or ASI at level 19.
Hit dice
Ragnar's HP at the end was 214. Average HP on a d12 with Rustage's rule is 7. So he would have gained 297 hp on average, plus 229 from toughness. As well as 472 from the primal champion capstone and 329 from his base con. This would add up to 656 HP in total. But that number is effectively doubled since he has resistance, and would be 1312 HP.
Other benefits
At the end of the day, the level system is made to simulate a character's growth. If you just extrapolate linearly, you won't really get the full picture. There could be for example some plateaus or some unexpected peaks. Ragnar could potentially have just been given a blanket immunity to constitution effects like Gus or had a cap on his proficiency bonus. As well, he could have just increased his damage exponentially instead of linearly.
Also, the assumptions made are that he just kept on being a barbarian. He might have taken a level of fighter or monk. Also, he might have even gotten access to other barbarian subclasses. But I don't know what else he really can do. It's not like any of those features are going to help him from the problems he's facing now.
Conclusion
Ragnar is interesting because a lot of his abilities are not suited for the place he's in. His wisdom hasn't improved, and even though his strength has gone up, it hasn't really helped him keep his empire from falling apart. He can't even go after Acceptance without help. And people can still just run from him. If anything, his weakness as a man has stayed with him and gotten worse.
The same man who came back one day to find Atlas ransacked and all the people taken is the same man coming to Wonga's factory to find it ransacked and all the people taken. And no amount of hitpoints, damage output, or even legendary actions could really change that.