Any advice on taking preorders?

There's nothing inherently unethical about taking preorders. Tesla's controversial business practices aside, pretty much every car manufacturer takes preorders, not to mentions books, games, etc. Where preorders get unethical is where people start selling things that don't exist or they can't reasonably deliver on, or when companies don't have sufficient financing to produce the good without the preorder sales (since then all the money goes right out and you can't return people's money if you don't deliver).

To be honest it sounds like this may be the case for you, since you say you don't even have the prototype, let alone the final device so you really have no idea when it will be ready, what the bill of materials is, how much it costs to manufacture, etc not to mention real inflation and supply chain risks on a 6+ month time frame, given the state of the world.

That all being said, why do you not want to crowd fund? It seems like it's better suited to the speculative, in development nature of your project where there is significant risk of not delivering as described, not delivering in a timely manor, or even delivering at all. While people get upset when crowd funded projects fail, they aren't nearly as upset when they purchase something from a company and it goes under before it ships.

I wonder if at the heart of your question, there is another core problem which is that if you need either preorders or crowd funding to finance the initial run of hardware, then you really don't have sufficient financing for your hardware ambitions, and that's going to make it very high risk for anyone ponying up money either preorder or crowdfunded. Perhaps this issue would be better address by talking to banks about lines of credits or investors about raising addition capitol. Maybe I'm off base here and money is not the issue at all and you just really have idea how many to order, but if that's the case you really need to invest in hiring some kind of business analyst who can help you develop models that will tell you optimal numbers to buy given lead times, expected market size, margin, etc.

What is the difference between crowdfunding and preorders? I think it would be a mistake not to crowdfund or use preorders unless you have significant venture backing and a sticky, viral consumer product where you can somewhat accurately forecast demand, or you anticipate selling out at high margins.

Don't waste your money building a product that potentially no one wants. Your crowdfunders are often the best feedback loops you'll get as a hardware startup.

Given that you have thousands of potential users and a working MVP, couldn't you get an investor to fund the initial production run(s)? Given a sound business plan and market projections, it might be an attractive proposition.

With pre-orders you are contractually required to deliver. Failing to do so could have some adverse outcomes. Crowdfunding comes with less legal risks.

If you arent in a position to completely refund everyone, don't do it.

If you need the funds to afford the manufacturing, take smaller deposits instead and use that money to negotiate a line of credit with a bank.

How will you accept payment? Most processors will freeze your fund if they find out you are taking preorders and not shipping out actually products.

Just take pre orders for your product in tiers make initial customers and scalpers pay a premium.