Is it possible to allow sideloading and keep users safe?

You are owner of the hardware, user of the software.

Okay, but as the owner then I should at the very least be allowed to load my own signing keys for the boot-rom to load other software. Like what if I want to run/port linux to the device. A locked down boot-loader deprives me of full enjoyment of the use of my tangible property.

That is totally fair.

...but we're conspicuously not implementing that feature. So take it, leave it, or build your own phone.

By the way, if you do go down the route of building your own phone, pedophiles, drug dealers, and terrorists will use it, and you're now on the hook to do something about it.

...Back to square one.

That silly pedophile point is getting old. Most victims of pedophiles are part of their own family or social circles. In a majority of cases no phone or even the internet is involved to commit the crime.

We could ban the internet completely and minors wouldn't be any safer.

In Catalonia they're now arresting people with GraphemeOS on suspicion of being drug dealers because only drug dealers would use secure phones.

Which is a great advertisement for GrapheneOS.

No they're not. Please cite a single source of this happening.

There were unofficial statements made by third party android news sites that Catalan police claimed that "Every time we see a Google Pixel, we think it could be a drug dealer"... which, while obviously wrong on its own, is completely different than claiming they are actually arresting people on suspicion of drug dealing merely by using GrapheneOS.

Please stop spreading misinformation.