Well... be aware that some people will disagree with me on this, and it's just my opinion.
Of course, the first thing to say is - this only applies to the unwashed masses. I wouldn't jailbreak if I were a politician, or if I was a journalist working on sensitive stories or living in a dangerous country. Or if I were, say, a billionaire. Basically anything that makes you, as an individual, an specific target for somebody. 😁
It seems obvious the security of your device is somewhat decreased when it's jailbroken. Now the degree to which that matters is arguable. I would think that people who are mainly interested in changing appearance aren't putting themselves at too much risk, assuming they're using well-known and established repositories. A tweak which alters your Control Center is probably less of a risk; one that affects your lock screen probably needs to be vetted more carefully.
You do want to be especially careful about tweaks which run daemons. For instance, if you enable ssh - I would suggest you leave it off most of the time, and just enable it when you actually need it (there used to be a password issue with the ssh daemon where there was a maximum effective character length of 8; I don't know if that's still the case. Also you need to change the default away from "alpine" for both accounts.)
When I've jailbroken, I've mainly been interested in customizing my user experience. Jailbreakers had the ability to customize the Control Center toggles long before that came to stock iOS, for instance - and even now have more options. Actually one tweak I really like allowed me to disable specific toggles when my phone was locked.
I actually think the current "semi-tethered" jailbreaks are more secure than the old "untethered" jailbreaks, even though they're less convenient, because they don't compromise the boot cycle at all. When your phone power cycles, it's as secure as anybody else's because the base OS hasn't been altered - the jailbreak isn't injected until you have had a chance to unlocked your phone and run an app.