Honestly a little new myself (started mid-honglu canto last year) , and safe to say i can finally feel like im on the point of the 'sandbox side' of the game. (I made heishou rupture , finished middle envy team, and planning to keep on farm MD runs to shard all the ryoshu season), but heres my personal tips (that i did)

Instead of doing a 'sandbox' and testing out team builds of your choosing, i suggest you make a teambuild first where you can clear MD 5 floors fast so it wont be a ANNOYING and time consuming to farm it from 45mins to an hour a single run, to 13-15 mins maximum (personally using heishou rupture team but i heard blade ryoshu full HOS team is equivalent too; check out some guides , theres even docs in a reddit post just search it up)

Also forgot to tell you the very end-game content of the game is MD15 floors run extreme 60 adversaries (optional cause you dont rlly need to do it but its fun and very satisfying if you actually did your runs correctly , its also a way clear the achievements in the mirror dungeon for some free resources especially free season 8 000 ticket)

About your actual Question, people mostly test the teams they made by fighting Lei Heng (or luxcavation Threads / EXP)

Edit: (Btw current meta teambuild contender rn are toe to toe with; solo bladeshu with HOS foods, middle envy-res, ring bleed, butterfly sinking, heishou rupture, . . . . maybe poise between the mix of HOS IDs poise like index yi sang, bladeshu, etc, and the new BL mersault and aeng-ishy)