Do we have are any hard data on deliverability differences in 2020?
I certainly feel like these companies add technical risk. My current provider (SendInBlue) automatically adds link-tracking with redirects, but this service seems to have poor uptime, meaning the links in my emails sometimes freeze at the redirect stage, which is functionally equivalent to breaking.
However, it's that they can help with handling the bulk sending properly especially with queuing and scheduling.
Re: link tracking, I believe you can always turn this off. Even for open tracking. Explore the settings and you should find where.