You will probably get a better rate on your primary residence.
Zillow mortgage will give you an idea of what the cheapest online lenders are offering. You can then apply through zillow or just contact the company directly you find on Zillow through their separate website.
It sounds like a HELOC that is used as needed rather than a cash-out 500k mortgage may suit you better.
Double check this, but I believe keeping the loan 100% for renovations and other housing related expenses allows the interest to be deductible.
Rate increases mean refi activity is down like 80%, so it should be a smooth process with the lenders being slow.
