Interesting build. Tips:
Look for items with 100% ED and get at least 3 of them for higher DPS as that increases your DMG hugely.
Yes poison is nice but you'd need Plagued to make full benefits of poison dot and poison cloud along with Blight or you can also have a look at crushing flames with Inferno set affix to speed up fire dot with crushing blows and faster bleeds, ice+ Frozen set affix or Shock + electrocution or even arcane + Arcanist. Or combine elements.
Weaken at 100%+ at least is very good on enemies since you'll end up dealing almost twice the damage potential and immune enemies become useless. Also very few enemies would be able to reduce your DMG below the factor of 1. There is of course Ignore Resist but weaken just happens to be stronger with a high enough amount. Ascendant also does a similar purpose with the arcane debuffs. Weaken and Ascendant combined is a very good combo.
Build looks so far quite decent. I think you knew but the Multi Attack and Extra Attack is great on Flintlock and primary skills in general. Read or me explain, it basically can add extra attacks and Increase your Mainhand DMG% by ×2 or ×3, increasing overall DPS from the flintlocks.
Attack speed is also extremely great with Flintlock and other primary skills for faster DPS output aka APS or attacks per second. If you happen to find Epiphany Bonus, you can use that to increase total DMG by 70%.
This can be achieved by crystal affix or epic affix, Hunger set and Angelic set affix (all of which increase APS). Angelic is available on amethyst and increases APS aka attack speed by 25% for skills that cast below 1 second cooldown.
As far as mythics go, I'd say cosmic orb like you choose , a chest mythic that suits your build. say Sureshot for guaranteed deadly strikes and Crits on full hp enemies , brutal mythic to buff deadly strike to ×3 crit DMG instead of 2× , ruptured if enemies can bleed from your attacks or even Exposed. Sure flintlock doesn't pierce but with the 50% pierce chance mythstone and 2 of them, you can pierce very well and that pierce would be deadly. I mean unless it already pierced.
Lastly as far as chest mythics go, definitely go with Skilled Mythic since that increases your flintlock MH% by 50% or any primary skill for that matter , which goes well with hero points and multi attack. Extra attack helps too.
Sanctuary is highly recommended for survival when dying so you can recover. For resource mythics, it's all up to you.
Fury is a good resource mythic with primary skill since no fury will run out when spamming primary skill but you won't gain extra DMG, that buff is for secondary skills (unless you use them occasionally). Energy is a nice one since it gives up to nearly 100% increased DMG when it goes down but it also regens fast (although it's not super easy to achieve). Alchemy is a bit complicated and may take some slots and testing to understand but it does work with primary skill builds just as well (but energy is probably easier or even fury).
As far as EXP farming goes, I'd say don't go above floor 200 as I've personally witnessed a decreased exp gain from enemies compared to enemies in floor 200 and floor 100 .
Other ways to increase DMG. Push the Limit because it increases your MH% and OH% and that means flintlock NH is increased by 50% as if you added a free +10 hero points to it (my simple analogy) ; Barbarian increases DMG by 50% but at a cost of MP which is costly but could work well and can work very well ; Glasscannon cuts your HP by 50% to increase your DMG by 50% but HP isn't a focus at higher floors.
To survive better at higher floors, you can look into higher dodge. 50% dodge, especially with stealth since stealth doubles that to 100% and with hero points, it can last a few seconds. Smoke bomb is also a good way to increase survival since it can manually dodge an attack and damage enemies while blinding them.
Block is another affix you can add to block incoming damage but it will only appear on your offhand weapon. Crystal affix block will only appear on offhand such as your bomb but that's fine. 45% block + 30%+ dodge helps as crystal affixes.
For set affixes in terms of DMG and overall winnings of higher floors : Pathfinder to sync with dodge and higher dodge means higher DMG; Demonic to increase DMG by 100% when enemies below 25% HP (it is like Relentless Talent in some way) ; Momentum + Move speed and move speed can be achieved by legend affix and skills such as coat, stealth , vault hero points , sprint , etc. Great with Adventurer set momentum is.
Others include permafrost for ice ED%, Plagued to increase poison dot DMG per enemy which is useful with poison cloud+ Blight and elemental critical , Crushing flames with fire elemental Crits combined with Blistering along with Inferno set , Frozen set to go with Frostbiting and ice for the exponential DMG explosions when enemies die for huge spike DMG , Arcanist with ascendant and weaken builds to increase ED% and even potentially Electrocution + shock with 1+ stack Debuff. Electrified exists but it's not really worth it for shock DMG.
You can even combine elements for cool effects and increased DMG as well. Vampiric Touch is a good set affix for increasing bleed DMG in high DMG bleed builds and it goes very well with Ruptured mythic . Bleed in general is very deadly but increasing bleed DMG by 125% like vampiric touch does is even better.
Another way to increase overall DMG is Criticals and potentially deadly strikes. Crushing blows help at higher floors when you can no longer increase DMG and you cannot change or upgrade the gear to make enemies lose HP so quickly before you can finish them off (even better with crushing flames and fire).
Criticals already increase DMG by 50% on its own but with 350% crit DMG as the cap, that means your crit DMG can be over 400% the normal damage . With 45% crit chance and base 10% crit chance, these Crits will happen so often and represent a majority of DPS. Combined with at least 20% or 30% deadly strikes or higher to increase the crit DMG by a further ×2 or even ×3 if Brutal Mythic equipped.
Other than that, you can make so many cool builds and unique builds with the advice/assistance I gave you or tips.