@Jingru SuiYes, you can rollover a Roth 401(k) over to a self-directed Roth IRA.

You could rollover or transfer to a self directed IRA, the funds stay in a qualified retirement account and then your SDIRA would purchase the property.

For costs associated with unit rollover like new flooring or dry wall repair, are you placing those in your OpEx (impacting NOI and sale value) or in CapEx (not impacting NOI and sale value).

2- Is there a way to roll over the Roth SD-IRA to a solo 401k account?

I have invested in corporate 401ks and have rollover IRAs.. 

You may avoid the taxes if you deposit the funds in an eligible retirement plan (which includes anIRA) within "3 years and a day" of the date of the COVID-19 distribution (note: compare to a 60-day rollover).

You may try and look at a section 85 rollover and see if it's possible given your situationhttps://www.marcil-lavallee.ca/en/resources-tools/monthly-newsletters/list-per-month/291-section-85-tax-free-rollover-to-corporations

Perhaps I roll over my 401k if I make it a full time gig, but seems to be no rush in that.

You may avoid the taxes if you deposit the funds in an eligible retirement plan (which includes anIRA) within "3 years and a day" of the date of the COVID-19 distribution (note: compare to a 60-day rollover).

Look into options to rollover into amortized payback or growth.Offer long term retirement funds that look similar to mortgage amortization schedules.Explore options to donate portions of the interest gained to provide community support to help rehab homes with people who need help, but can’t afford it.Create regularly updated, highly visual progress reports for the various remodel/ renovation projects.