Well the spreads are kinda weird, why isn't the Latias max speed max HP? Reuniclus needs speed to outspeed toxapex as well.

This team also loses to Koko and Zone, since it can freely spam Volt, weakening your team esp if rocks are up (that's why that kind of team usually run Gliscor). Zone is especially a dead sentence, since it also removes your defogger, making volt spamming even more dangerous. Not a fan of bunker pex as well, personally ; Toxic is very valuable, and make pex far less passive than with bunker. Chansey usually exclusively fit on MSableye stall, since how much it hates hazards. Also taunt cm fini immediately wins against that team.

It's difficult to say everything that needs to be known about fat teams to build them - and anyway, i can't pretend knowing everything about those kind of teams (tbh i doubt anybody could), but here are some good tips:

To build stall, or bulky archetypes in general, you need to ensure that you either have a consistent counter to each threat in the metagame, or at least ways to prevent it from getting opportunities to break your team, which means you have to choose your mons carefully, so each mon cover other mons' issues (stall is sometimes considered like a 6-mon core for that reason). You can't stack fat mons and expect it to work, especially considering that you need to cover as much threats as possible, while ensuring being able to make progress against most playstyles.

Using the viability ranking or the usages stats as a threatlist generally is useful, to see if you have a solid gameplan against each threat in the metagame. Ofc knowledge of the metagame generally is helpful ; most cores/teams involving a breaker are made to make that breaker work, and understanding how those kind of structures function is important imo. Pivots, terrain support, trappers, or just stacking things that share a common check to overwhelm it are example of those strategies. If you have a counter to each mon of the opposing team, that doesn't mean you're have a good matchup!

That knowledge is made by playstesting your team : the more you build and play your teams, the more you'll understand how the meta (and the game in general) works. That point isn't only valid for stall, but every playstyle, obviously. Keeping and watching your replays (especially the ones where you lost) is especially useful. In other words: if you want to learn how to build solid stalls, you have to build and use bunch !

I wrote this at 2:30 am so i'm sorry if this explanation is a bit unclear or messy, but i hope it helped ! If u have other questions feel free !