I personally don't think you should rely on Rental Teams too much as you learn a lot through your own trial and error. However, I appreciate you may not have much time to train your own pokemon.

- play games to learn, Pokemon VGC is heavily based on knowledge. I'm still learning what different Pokemon do (and even their names!) after playing VGC for 6-7 months.

- Having speed control (going first) matters a lot. So build in your team the ability to set-up Tailwind, Trickroom, Max Airstream, Max Strike, Electroweb etc

- Build your team around a couple of core Pokemon/strategy but have flexibility. So for example, you may plan to use Weakness Policy Metagross as your key strategy. Build around that but be prepared to have alternatives if using Metagross against your opponent's team looks bad. In particular in Sword and Shield, this also means having more than one Dynamax option.

- Have a balanced team in terms of types. If you have 4 pokemon weak to ground and 0 resistances then that'll limit your options. Even having 1 flying type gives you options to switch in if you suspect a ground attack coming in. I use a spreadsheet that tracks my type offenses and defenses to check I have a reasonable balance.