You can use $500,000 of your $800,000 capital loss to directly offset your $500,000 capital gain and then another $3,000 to offset any other type of income. The balance of your loss will be carried forward to the next tax year.
Your investment (margin) interest is deductible but subject to strict limitations.
See https://www.irs.gov/publications/p550#en_US_2021_publink100010206
