The Google slides template would be designed inspired by your branding. I propose the below

Size: Widescreen (16:9) or Standard (4:3) as per your preference
Fonts: As per your branding /website ensure it is Google slides compatible fonts
Template Layouts:
1) Cover Slide
2) Divider Slide
3) Content Slide
4) Thank You Slide
I am open to incorporating any additional layouts as per your needs. The estimate for these would be shared separately.

The template will be designed to cover these aspects
1) Essential aspects like colors, style themes, typography, etc properly fixed
2) The template is flexible to work with
3) Slide master has the key reusable layouts
4) Default elements fixed (e.g., title slide, headers, footnote, source, etc.) are consistent and are aligned correctly across the template.

Google charts have different logic and need more time to fix. My suggestion is to use PowerPoint charts and import them as pictures to Google slides.