This is correct.
What you’re looking for here is a Duo/Family plan pretty much so that way you can have several accounts with their own passwords.
You missed the Xmas deals 
for good opportunities but some other ones might come soon enough.
If the goal is to get your family to use several free accounts, this is where the family plans kick in. If it’s for your own use case, I still think that folders are more than enough, I mean it’s hard to know because you haven’t told us what do you plan to do with them.
I meanwhile know that if you buy a family plan and then downgrade it to a free one, they will “soft-delete” the accounts meaning that they will add limits to the accounts themselves (think Proton Drive’s free 5G rather than a duo 2TB for example) so that they do match a free tier.
If you’re still above it, they will give you some time before they start deleting the data if you’re still over the free tier. The amount of time might depend on how long did you pay for I assume? If you buy a yearly family plan, you’ll get a yearly deadline (at least, this is what I got).
But if you get a shorter one, I am not sure if it will actually be shorter or still 1 year.
SimpleLogins will meanwhile still stay active, you just won’t be able to create new ones.
This is hence a soft-delete where they try to not make it too disruptive while still giving you some time to move away gracefully.
And again, we don’t know what you want to achieve here but if the goal is to get some accounts under the radar while going against their policy of having several free accounts under the same roof, don’t be sad if they do one day delete most of them.
Hence, depending on your goals: consider paying for a family plan rather than hoping that you will never be spotted/punished so that you could avoid yourself a big waste of time/headache down the road if[1] they spot you.
BAKKA UPDATE. 
If this is your real intention, why not use folders/labels and maybe even filters? They are very flexible, allow for proper segmentation and will make everything tidy + organized so that some stuff doesn’t notify you, or goes straight into trash, etc.
Having to hop between several accounts with MFA sounds like a huge pain and will mostly lead to you having mistakes/frustration sending yourself emails from account A to account B.
If you want to get serious about your work and need some public branding, consider buying a domain name and plugging it to a SimpleLogin with a catch all or just a SimpleLogin (it can act as an email server-only too, without the aliasing part of it).
This is not a healthy thing to do, what the average person does and how you plan things should not be 1:1. Organize yourself given your needs and not what other people do.
Email can be self-hosted but it is very not recommended.
Paying for email sounds like a very fair and sound thing to do given how important it might be + how hardcore and technical it can be to self-host.
Hence, this has nothing to do with an entertainment/hobby (like a Netflix) subscription and should be treated more like rent, electricity or food groceries IMO.
TLDR: then, folders + labels + filters it is.
Peace of mind, proper compartmentalization and flexibility all in one for the fee.
And if the free Proton mail tier do not suit you, then you need to decide if you value your privacy enough or care more about the fact of not having a subscription.
You can’t always have both worlds for the free while keeping your data for yourself.
I mean: self-hosting is the final-boss solution[2].