@Alexander Quan Generally, a Solo 401(k) can accept tax-free rollovers from your former employer's 401(k), even a Roth 401(k). (But not a Roth IRA.) It sounds to me like unless there are some missing facts here, or I'm misinterpreting your statement, your "tax person" who suggested that rolling your former employer's 401(k) into a Solo 401(k) would be a taxable event isn't aware of the rules.
It sounds like you are eligible for a Solo 401(k) since you have self-employment income from being a real estate agent and flipping from which you can make contributions into your Solo 401(k)
And you can certainly buy properties in your Solo 401(k), so your goal of using your 401(k) funds ASAP to start acquiring properties is very doable assuming you qualify.
Do you really need these funds before retirement? It sounds like you have a good career in pharmacy and other streams of income on top of that, so why take the net worth hit of taxes and penalties if you don't have to and can reach your real estate goals within the Solo 401(k) while enjoying tax-deferred wealth building?
The cumulative effects of paying tax on your profits every year is underestimated by most — we're talking tens if not hundreds of thousands of dollars for even a modest investor over 30 years or so — and being able to defer those taxes for decades so as to reinvest all profits will be a boon to your net worth and financial position.
Also, be wary of "flipping," per se, in your Solo 401(k) as you could come under the purview of the UBTI rules, with the result that you would have to pay tax, and the whole point of investing through a 401(k) is to defer tax. Buy and hold is generally safer in the Solo 401(k), or possibly fixing up a property, renting it for a time, and then selling it so it's not "flipping," per se.
And of course you always have to watch out for the prohibited transactions and make sure you keep good records for your accounts and work with a banker who knows what they're doing.
@Dmitriy Fomichenko is an expert in this space. He happens to be in Orange County and could probably talk you through your options.