KUVA ELEMENT PROGENITOR: TOXIN
I've tried and tested other elements, as the status chance really is low it is advisable to stick only to a single(combined elements such: viral, gas, corrosive) and not go to the multiple elemental mods build as this weapon got too low status chance percentage proccing multiple elements at the same time ain't gonna cut it.

ELEMENTS TO AVOID:
Impact - if you are thinking of building [Kuva Tonkor] with Internal Bleeding, even with 60% Impact Valence, impact status won't proc a lot maybe once every 10 seconds from my testing. From my personal analysis Hunter Munitions is far superior to this Internal Bleeding for this weapon.
Magnetic - only good for primers and this ain't one for sure.
Electricity - elemental combination is very limited and is not helpful most of the time.
Cold - weirdly enough even with the 60% max, this cold element proc more compared to 60% Impact. However this is not very versatile, it procs but not enough.
Radiation - I haven't tried radiation but I'm thinking it might be good considering weapon is aoe.

Note: If you don't have a riven place [Vigilante Armaments].
Suggested riven stat: Critical Chance, Critical Damage, Multishot, +Weapon Recoil (Good in general, harmless negative plus all desirable positive stats)
Alternative riven stat: Multishot, Projectile Speed, Reload Speed, +Weapon Recoil (For General Use, good on Void Fissure Missions, good for spamming)
Quick Tip: Do you know pressing "R" on the keyboard after firing a shot would reload the [Tonkor] faster and removing the delay on the auto reload.

Very versatile build forma selection and forma placements are strategic.
Elemental combination would be very versatile with the Toxin Progenitor. Meaning you can have Gas, Corrosive or Viral with this build.

I recommend using [Critical Delay] over [Point Strike] with one click one shot mechanic fire rate is really not needed.

I personally use Fractalized Reset(weapon arcane)+Arcane Rise(warframe arcane) over Primary Merciless, but this post is just a general build.
If your warframe is built for shield gating, fractalized reset would make [Tonkor]'s reload in instant.
Quick Tip for Fractalized Reset: It says on it's description that you have to cast a skill to activate it, though by pressing 5 or going to your operator mode/transference would also trigger Fractalized Reset making your reload +240% faster.

If not a fan of Primed Bane Mods just replace it [Serration] or [Amalgam Serration].

[Primed Magazine Warp] can be replaced with [Primed Fast Hands] Depending on your preference.
If you want to slot in [Hunter Munitions] on this build, only way is to replace 6th slot from [Primed Magazine Warp] or [Primed Fast Hands].
Though I would stick to [Hunter Munitions], faster reload mods are only good for quick missions like Void Fissures, but it is better to have Hunter Munitions on Survival/Endurance mission specially on Steel Path.

Lastly if you feel you can manage your ammo efficiently then you can try inserting [Terminal Velocity]. This would only work if you are not using [Primed Magazine Warp].
If it come to the time that this weapon got a max ammo nerf just like what they did to [Kuva Bramma] and [Kuva Zarr], it is always best to use [Primed Sniper Ammo Mutation].
I personally use it already but you can replace it with [Vigilante Supplies] for that 5% chance to enhance critical hits.

BONUS:
On void fissures mission there's a gimmick whenever you picked up 10/10 reactants a certain ability will be unlocked like unlimited ammo with no reload needed. With that one click one shot mechanic and with the current ammo economy updates from veilbreaker, ammo wouldn't be an issue unless they nerf it again.