I've been linked this review of email service privacy previously. Obviously everyone has their own threat model and you may not agree with theirs but I think it's worth a read for services you may be interested in (or just the summary).
I've used Mailbox.org and don't care for their interface at all, but it also matters not one bit as I use Thunderbird exclusively to interact with my mailbox, as you plan to. I haven't had any problem with spam but I am very picky who I give that address to.
My personal opinion is that the provider should not matter - your address should be a privately registered domain and your emails should be end-to-end encrypted. Then your mail provider is little more than a forwarding server and the most crap one is not much worse than the best.
If you find an answer to that please let me know