18.5 is easily one of the most fun touhou games I have had the pleasure to play through. This one is far different than most, focusing on randomly generated stages and a random boss at the end of each. In these stages you earn two types of currency, which are used for ability cards, each granting a special buff, passive, or move to use in that run. There is also the addition of the Magic Circle, which will freeze bullets for a period of time, allowing you to attack patterns in special ways.
18.5 is an absolute blast, its super fun to just hop on and play a few rounds, hop off, and then come back a bit later. I do think it suffers from being a bit bare in content, there could def be a few more enemy patterns and bosses, but other than that super fun.