"Covid" is better than "Covid19"
1) It's shorter.
Shorter domains are better. Also, covid is replacing covid19 as the go-to term when talking about the disease. In the beginning, people tended to say more "he's got covid nineteen" as the disease becomes a more present thing people just say "he's got covid" as usually happens when things become more ingrained in our lives.
2) It's more evergreen.
"CovidGuide" will be useful for any other Covid mutations in future years (Covid21 Covid25... whatever new mutations)
Covid19 stands for COronaVIrus Disease 2019 because that was the year this coronavirus disease started. Future diseases from the coronavirus family will be called "covid" with a different prefix or suffix but with "covid" being the generic term that encompasses all of them.