My three week old startles regularly and wakes himself up if his arms aren't swaddled. It's very hot (high 30s, low 40s) so I worry about him overheating when swaddled. Are the 0.2 tog sleeping bags with arms inside safe? E.g. woombie grow suits?

Are you using a muslin/cotton wrap for swaddling? This is Red Nose recommendation, as these are light & are unlikely to add warmth to baby as an extra layer of clothing would.

Red Nose recommends parents and care givers use lightweight wraps such as cotton or muslin (bunny rugs and blankets are not safe alternatives as they may cause overheating).
Red Nose does not recommend the use of swaddle bags when baby’s arms are enclosed as an alternative.
Make sure that baby is not over dressed under the wrap. Use only a nappy and singlet in warmer weather and add a lightweight grow suit in cooler weather.
https://rednose.org.au/article/safe-wrapping-or-swaddling-when-to-stop-and-how-to-select-a-safe-sleep-bag

Are you using any extra bedding (eg blankets) for baby?
How much clothing does baby have on under swaddle?

The best way to reduce the risk of overheating is to dress baby for the room temperature & to keep baby’s head & face uncovered.
You may find these Red Nose links also helpful
https://rednose.org.au/article/protecting-baby-from-overheating-during-sleep
https://rednose.org.au/article/how-much-bedding-does-baby-need