Havent really messed with BLU as it seemed really tedious to get going and needing others to help you. But ye ventures drain at solid rate with 5 and I dont spam quick ventures only, the normal 1 cost ventures are very much worth as well as some of the materials are very pricey + they are great for stocking up mats. Like after patch hit, new craft gear -> mass amount of materia needed -> cubus flesh prices climb like hellOh yeah poetics I can see, goetia are way too valuable to waste on ventures imo. If you have a BLU you get a fair amount of ventures every week doing the carnival, it's like 540allied seals and 490poetics or whatever the numbers were, the allied seals buy you 130ventures a week and you can throw the poetics on top for a few more. Granted that won't cover your needs if you're spamming quick ventures on all your retainers but it helps quite a lot for less than 10mins of messing about.
I still would desynth, it's more money most of the time, at least until the price of sublimes really crashes.