The D.C. Superior Court Probate Division has some good information about probate: https://www.dccourts.gov/superior-court/probate-division
A helpful website for Maryland residents with probate and estate information: https://www.peoples-law.org
Information on the District of Columbia estate tax can be found here: https://otr.cfo.dc.gov/publication/d-76-estate-tax-instructions-booklet-and-computation-worksheets-2022-and-prior
Information on the Maryland estate tax exemption: https://marylandtaxes.gov/individual/estate-inheritance/estate-inheritance-faq.php (This site discusses how portability is available for Maryland residents. This means is essence that a married couple gets double the exemption).