Don’t respond to spam feedback loops from major mail providers and you’ll quickly get blacklisted no matter how much reputation you built up. This could happen at any point and doesn’t require that many people to mark as spam.
It’s worth paying someone if deliverability is important.
Deal with the spam feedback loops - they exist for a reason. You have to treat an FBL email as someone hitting the "unsubscribe" link (though it might also be an indication your unsubscribe link has to be more visible or your unsubscribe process less obtuse). Some off-the-shelf software can monitor your bounce and FBL messages and automatically process them as such.
Getting blacklisted is a temporary state and I maintain it's easy to recover from if your IP's and domains had a good history. I've experienced the IP's that email hosting services send my mail through being blacklisted and then waiting days for them to deal with it versus being able to contact blacklists on my own behalf to resolve things same-day.
I used to pay one of these services $xxx/month and experienced delivery rates around 60-70% that they couldn't do anything about other than offering me a dedicated IP for a lot more money. Moved to my own mail server with an OSS transactional email system and delivery rates shot up to 99%-ish while eliminating the email bill.
It’s worth paying someone if deliverability is important.