You are half way there by using a commercial registered office in Washington State, which is common place and meets the primary requirement of providing a physical address at which legal process such as a summons commencing a lawsuit or a legally required notice can be hand delivered.
While Washington State does not allow for a mere private mail box address to be used (unlike most other states), there is something a notch up from that which would probably pass muster.
An Office Membership
Firm like this one (which I have used in the past more than once) have a physical office suite shared by many firms with a common receptionist who is a human being on site who answers phones, greets guests of people who have contracts with the firm, and makes sure that the break room, bathrooms, copying machines, staplers, lightbulbs, door locks, heating and cooling, etc. are working and calls someone to fix them when they don't, if it involves more than simply restocking supplies. Often this is a suite in a larger office building in a prime location.
Contracts with the firm range from a full fledged space rental on a long term basis, to a right to use an office a few days a month, to a right to use conference rooms a few days a month, to a right to use offices or conference rooms in the suite on a pay per use basis at a higher rate. Everyone with a contract also has incoming calls handled by the receptionist that are forwarded to any cell or landline desired once answered if a caller is doing more than leaving a message, receives mail and packages for them, allows them to use the office suite's WiFi, office equipment and break rooms during office hours, and provides them with some way (sometimes optional) to use a physical conference room, or work space (private or open) at the negotiated rate. Contracts are typically month to month unless a longer term is agreed to for a lower rate per month. The full fledge private office space option is sometimes called a "serviced office" and the lower end options such as renting space in the suite to use by day or hour version is called an "office membership" which can be quite inexpensive (the linked firm has basic options as cheap as $84 a month).
Larger chains permit you to have an "office membership" that can be used at any of their locations even though you have one primary location.
I strongly suspect, although I don't know with 100% certainty, that an office membership at such a location would suffice to meet the principal office requirement without being a "private mailbox".
Using A Friend or Family Member's or Professional's Office Or Home
The cheap and dirty solution before such firms existed was to have a friend with a regular office agree to put the name of your firm as well as his or her own on the directory for his office building and designate a corner for you to work if you are ever present in case anyone asks where your space is at the location.
Many sole practitioner and small firm attorneys and accountants have historically offered this service to their clients as a way to generate a little regular cash flow for very little effort, or as a perk of being a client, although it is less common these days.
Another common cheap and dirty solution is to have a friend or relative agree to use their residential address as the principal office for your business.
Delaware And Other Options
Delaware requires a registered agent, who may be a commercial registered agent with a physical registered office, but does not require a physical primary address for the business in addition to the registered office.
For what it is worth, many other states have requirements that the same as Delaware in this regard (e.g. Alaska and Maine) and some of them are considerably less expensive in terms of filing fees and registration renewal charges.
Unless you have a publicly held company, you anticipate sticky internal governance litigation, or you want to do something really unusual that most states forbid (like eliminating all fiduciary duties that managers owe to the company), there is no good reason to prefer Delaware.