I use Buddy Belts & I also have a Choke Free harness. Maximo sells BBs here & LilSis sells the CF...both at discounts for YT members.

I'd say at 5 months, you can probably invest in a Buddy Belt. The CF harnesses have to be sized exactly & she might grow an inch or two around her girth. The BBs are adjustable - like a normal belt. They're expensive compared to the fabric harnesses, but I think they're well worth it.
Here's a link to both:
BB -
Buddy Belt Harness
CF -
Pet Collars and Harnesses that do not choke dogs and cats