I see several issues with your SMTP session.

First, gmail.com will be protected by SPF and DKIM and your message will likely be flagged as spam (or outright rejected) because it’s clear that you’re not sending on behalf of the real gmail.com.

Second, commands should be in all-caps. A server may accept or reject lowercase keywords.

Third, you need to leave a blank line between the mail headers and the body, so that part of your session would look like so: …

DATA
354 Go ahead
From: ...
To: ...
Subject: ...

This is the first line of text.
This is the second line.
.
250 Queued

Having said that, many servers will require an encrypted connection (SMTPS), many ISPs will block port 25 for residential customers as an anti-spam measure (so your local mail server may accept the message from your script but be unable to forward it), ESMTP should be preferred over SMTP etc.
If at all possible, you should use a full-featured mail library for this and use your ISP’s own mail server.

Doesn’t Pop_OS come with a sendmail command?

I agree in principle - on Windows it’s a bit of a cat-and-mouse thing between people building tools to disable Windows telemetry and Microsoft building ‘better’ telemetry. And don’t get me started on Edge. It really is time for the courts to force Microsoft to allow consumer choice once more.

Having said that, it does depend on what your objective (resp. threat model) is whether or not you consider Windows telemetry a problem. Microsoft will know that you’ve used this web browser for that much time, but not what websites you’ve visited (unless it’s Edge of course). It’s up to you whether that bothers you.

It depends on what you want to achieve.

Encryption (if done right) will protect you against people eavesdropping on your connection, but not against tracking by cookies, device fingerprinting or similar technologies. I.e Google, Facebook etc. will still be able to track your every move. A web browser with good ad/tracking blocking will go a long way here, but if technically feasible you’ll also probably want something like Pi-Hole to complement your browser’s ad blocker and also catch network traffic from other apps.

For better recommendations you’ll probably need to tell us about what exactly it is that you want to protect yourself against.