Avoiding the spam box

I have a fledgling newsletter with a couple dozen subscribers at this point (working on that!). I know there have been issues with confirmation emails going into spam, and even some instances where after confirming emails have still gone to users' spam folder. Any advice on how to keep this from happening? I feel like I'm following best practices in general, for example:

• Emails are pretty much all text excepting a header at the top with a logo. No flashiness or otherwise.
• I have an unsubscribe and privacy policy link at the bottom
• There's no verbiage in the subject line or otherwise that should trigger a spam filter (i.e. 'winner!' or 'Buy now!')
• The domain the emails are sent from is just a gmail account (not sure if that's actually best practice or not...)

You can review a recent message here if curious.

Would it help if I setup an email account with a custom url instead of using gmail? Right now, I'm just using the newsletter plugin in wordpress. How abut going through substack or otherwise, are emails generated from other platforms more trusted? (ignoring other advantages/disadvantages for the time being)?

Thank you for your thoughts!

edit: I just found a usefull tool to check spamminess of an email called mail-tester .com. I ranked a whopping 3.1/10... Luckily they tell you why, which means I have some work to do.
It also appears that sending from a free gmail account is not exactly ideal. Will fix that!