Tips for good numbers: dates work pretty well. Another thing you can do is use a simple cipher The simplest one is the one on old touch-tone phones. Then just memorize a word and convert it e.g. F-L-A-S-H-Y = 352749.

Or just use a password manager like bitwarden and keep it in there.

porkbun.com tells you the renewal cost. the cheapest is a series of at least 6 numbers with the .xyz domain.