Use 5 Azura Archon Shards with ARMOR BONUS:
This takes you from 1.146.710 EHP to 1.571.370 EHP (AFTER ALL THE OTHER BONUSES). It's a 37,03% increase.
Hitpoint shards would take you from 1.146.710 EHP to 1.346.500 EHP which is only an 14,83% increase.
(So ARMOR SHARDS are more than double as strong, even though we have a lot of other things to give us armor (Fiber, HC, AG, Scarab Swarm)
^(This is all calculated with tauforged shards, but even if using non-tauforged, armor will of course still be better.)

OVERVIEW/MATHS:

Okay, so let's look at some math (boring ik). This is using this site: https://hypothermia.cc/tools
Let's start off by looking purely at base STATS and the current MODS.
9.333 hp and 936 armor results in 38.452 EHP. (10x with [Adaptation] (This will apply from now on, even if I don't mention it))

Let's try swapping out [Gladiator Resolve] (This is the least beneficial mod for tankyness) for [Primed Vigor]. Then we have 41,933 EHP. THIS would however require the use of another Umbral Forma. And the Critical Multipliers will come in handy later in the build. The EHP difference isn't that huge, so I advise against [Primed Vigor].
Another option could be swapping Gladiator with [Saxum Carapace]. That leaves us with 40.356 EHP.... BUT we will again lose out of that crit multiplier PLUS we will get a lot of armor from other sources, makes this have diminishing returns....

Talking about the other sources. If you have 3 stacks of [Health Conversion], you go from 38.452 EHP to a massive 80.450 EHP!!!!! Even though the mod is a bit unreliable, just having a stack or two will outperform most other options.
Also with Arcane [Guardian] active and everything mentioned above you will have 108.449 EHP!!!!

There is Stone Skin from Unairu focus school. This adds +200 base armor additive. Resulting in 114.671 EHP

Then there are those Archon Shards. If we have 5 normal shards with armor bonus, that's +150x5 armor = 750 armor. Resulting in 138.004 EHP. Using Tauforged shards 225x5 = 1.125. Which would result in 149.670 EHP.

Finally we also have Scarab Swarm. This ability makes [Inaros] have +100% BASE armor. (base armor x multipliers)+additive sources (this is how it works), so its only +240 armor bonus :( but this leaves us with 157.137 EHP. Then if you have 9 stacks [Adaptation] you only take 10% of damage, so that's 1.571.370 EHP!
I do realize that not all buffs can be kept up at all times. But then again you also gain good life steal from Gloom, so some factors are unreasonable to calculate. But considering how easy you gain HP with Gloom active, I think saying we have 1.571.370 EHP is more than fair!

I also know Warcry is popular, but using it only takes us from 92.06% damage reduction to 92.31%. The LIFESTEAL and CROWD CONTROL from GLOOM is MUCH better lmfao.

DAMAGE:

Yes, now for the DAMAGE part (This is also where [Gladiator Resolve] comes into play) :O [Inaros] doesn't have any good abilities. Or does he? :O So on his first ability [Inaros] throws sand into the eyes of enemies in front of him (Im gonna put some dirt in your eyes). This opens them up to FINISHERS. Thanks to the new Melee Crescendo, you can then build PERMANENT 12x Combo multiplier :O
So you go in a mission. Throw sand in their eyes. Finisher kill them. Build your melee combo. And boom, now you have a melee that does MILLIONS :O of bleed damage on heavy attacks. Talking about the [Reaper Prime]! Reapers have forced bleed proc on heavy attacks, so this weapon is our perfect candidate, here is the build!
MILLIONS OF DAMAGE PER HIT

Now we have a pretty tanky and braindead frame + a weapon that will one-shot most enemies probably up to a couple thousand levels. Finally, Inaros can do something (Except if the enemies deal so much damage by then that he gets one-shot too lmfao)

ABILITIES:

Gloom for lifesteal and crowd control. However, Blood Altar heals in % which is amazing with such a large health pool, so it's a preference thing.

MODS:

If you find [Health Conversion] a bit unreliable, you could try to swap it out for: [Hunter Adrenaline] or [Primed Vigor].

ARCANES:

Arcane Avenger or Arcane Blessing (Avenger is going to be up most of the time (Since [Inaros] has no shields, thus taking health damage)) (no longer using Arcane Grace, because we have Gloom anyways (unless you just want to stand afk, which I advise against, lmao)) Blessing gives a total of +1.200 more HP, resulting in 170.319 EHP. 1.703.190 EHP with [Adaptation].
But the nice part of Arcane Avenger is that it gives critical chance % AFTER other calculations, making [Blood Rush], [Sacrificial Steel], and those Gladiator mods even better!

No matter what. You might think he is the tankiest because of his health, but in endgame, you mostly rely on CC or shield gating to survive, the enemies deal more and more damage the higher the level they are, for example:
-Inaros vs lvl 1 to 199 enemies : joke
-Inaros vs lvl 200 enemies : fair fight, you might need lifesteal to survive
-[Inaros] vs lvl 201+ enemies : [Inaros] 1 shot
(^Investing in defences only push further the lvl cap at which you'll get 1 shot.)

I think the only reason to play him is if you are playing "low level" missions (1-200) and just want to play a chill playstyle. Enjoy!

COMPANION:

[Dethcube] for priming. My Dethcube Prime Build AND Weapon (Viral+Heat)

FOCUS SCHOOL:

Unairu for bonus armor-

Build Showcase