No difference regarding inboxing capabilities. Workspace business accounts might have the slight benefit of having the custom domain in the address, therefore b2b could take you more seriously. That said, regular Gmail works for that purpose as well.
- Gmails use Google's own smtp servers. They all have the correct setups you mentioned.
- Instantly uses the same exact AWS vps and static IP for all your accounts.
- Gmass is browser based, therefore also uses Google static ip and smtp servers.
100k/day does need a solid infra though. It can be done with Gmail, Workspace even Outlook.. whatever you want.