Help buying corrective swimming goggles

Training for my first Ironman 70.3 and trying to sort out my swim gear.

Right now I’m using regular Speedo goggles, but I recently found out corrective swimming goggles are a thing and I’m looking to switch.

My prescription is pretty high: -5.50 sph and -2.50 cyl.

The options available to me in India:

1. Speedo Hydropure Optical (only goes up to -5.50)

2. TYR Corrective Goggles (available in -7.00)

3. First Lens Corrective Goggles (available in -7.00)

No other brands/options are realistically available here.

A couple of questions for people who’ve used corrective goggles:

- How much should I compensate for cylinder/astigmatism when choosing swim goggle power?

- Since these are spherical only, would going for a stronger power make sense?

- Is it better to go with the exact sphere (-5.50) or size up (-7.00) for better clarity?

- Any experience with TYR or First Lens optical goggles vs Speedo?

Would really appreciate any advice!