When I was a beginner I made the same mistake as you did: based my builds on full endgame gear (legendaries and impossible greens) crucible/SR speed farmers instead of the beginner builds available in the forum. If you still don’t know it, there is a beginner builds section in the build compendium from Stupid_Dragon (Build Compendium X (Forgotten Gods)) and a separate beginner build compendium from Ulvar (Beginner build guides Compendium (for AoM + FG expansions)). If you don’t have a deep understanding of the mechanics in GD, it is always safer to follow a guide from someone who does until you understand the reasoning behind all his/her decisions (gear, devotions, skill points).
Stupid_Dragon himself has a great up to date beginner guide on blademaster ([1.1.9.0] Beginner's Forgotten Blademaster), which I just looked closely and agree with basically all his gear/devotions choices (considering you don’t have access to better gear). When comparing his build (https://www.grimtools.com/calc/O2GEKnJZ) to yours, most of his gear choices would be better for you: his medal, chest, shoulders, amulet, belt, and relic. These are all also very easy to get. There is also a great medal from solael faction (https://www.grimtools.com/db/items/11575) that is vastly superior to yours.
The only gear pieces that you have that are superior imo are your rings, pants, and maybe the helm (debatable because you don’t have capped slow resist like in his build) and that is mainly because you should never recommend random drop legendary gear in beginner guides (especially barbaros pants, you got pretty lucky there). Also keep in mind that you already have 190% attack speed without the procs from the rings and that the cap for attack speed is 200%. Thus, in terms of attack speed, your second proc is not doing anything. A different ring (like this one https://www.grimtools.com/db/items/9025) would be a better choice for the second one.
Regarding your relic, Azrakaa’s Epoch shines on blademaster builds that are not skill point hungry (like belgothian’s set, because then you don’t need to max cadence) and that have high amounts of OA to proc the relic skill consistently. Your build only has 2800 OA, so a 33% proc on crit skill will be really unrealiable. Furthermore, the main point of the relic is to provide AOE clear to crucible speed farmers, which is not your case. Serenity, as you noticed, is quite a popular choice in cadence blademasters because of the +1 to all skills and the fact that is a good all-around defensive relic (works in almost every build). Since your build is starving for skill points, I would go for either Belgothian’s Carnage or Serenity (even if you improve your OA).
Regarding the flat RR discussion, I made a quick search on the strongest pierce blademaster builds and all of them have flat RR sources somewhere (you probably just didn’t notice). Examples: https://www.grimtools.com/calc/bVAXlY62 (has flat RR from gloves, as Stupid_Dragon said), https://www.grimtools.com/calc/mN491Yx2 (has flat RR from Tips the Scales devotion), https://www.grimtools.com/calc/D2p9JLaV (has flat RR from Tips the Scales devotion as well). If you want to keep both Azrakaa and Unknown Soldier devotions, then I would suggest copying the devotion map from this build from banana_peel: https://www.grimtools.com/calc/mN491Yx2. Reasoning behind it: you get both tier 3 damage devotions without losing the safety/sustain from ghoul and keeping the flat RR and little sustain from Scales. The only issue is you would lose Ulzaad, but getting Unknown Soldier might be worth it.
Finally, I would still like to point out that your OA is too low for a pierce blademaster build. The reason why OA is so important in this build is that several very important devotions are procced on crit (Living Shadow, Blades of Wrath and Assassin’s Blade). To fix it, appart from the gear suggestions, I would start by changing your skill point allocations. All top blademaster pierce builds keep anatomy of murder at 11/12 (sweet spot) or max it because of the cunning bonus and racial damage. Cunning double dips your damage by increasing both your pierce damage % and your OA, so the more the better on a pierce build. After that, field command and pneumatic burst are good choices to increase your OA further. I think both skills (especially field command) are better investiments than Fighting Spirit and Fightning Form. Even after the buffs, I would only cap Fighting Spirit on a less skill point hungry build like belgothian’s blademaster. Fightning Form is great, but mainly maxed in crucible speed farmer builds to increase the aoe. Both skills are good tbh, but there are just better alternatives. The builds that skip OA boosting skills on pierce blademaster probably have 3.2k OA+ already (two of the ones I linked have 3.5k)
OK this post got really long. Hope it was helpful.