I've been doing a combination of 1 (good reputation IP) and 4 (using AWS in my case). I switched VPS provider a couple of years ago - before that I had deliverability issues to anyone using MS 365 (despite jumping through Microsoft's various hoops etc) - since then everything has been fine.
I also have my outbound SMTP server set to send via AWS if my email includes a particular custom header (which it also then strips out before forwarding on) - which means for domains I think might have deliverability issues I can deal with this without needing to make a huge effort.
I also have my outbound SMTP server set to send via AWS if my email includes a particular custom header (which it also then strips out before forwarding on) - which means for domains I think might have deliverability issues I can deal with this without needing to make a huge effort.