There are several single player Magic The Gathering games, but I think only the first one from 1997 gives you full deck building flexibility. The sequels just have a bundle of pre-builts to choose from.
The 1997 version has one-on-one Constructed and Booster Draft modes, in addition to a roguelike-RPG game mode.
If you liked shandalar, you should check out forge like someone suggested in another comment on this thread: https://github.com/Card-Forge/forge
It's a free, fan made, FOSS game that implements even the newest cards coming out, and it has a shandalar inspired adventure mode for you to play, where you can defeat all color wizards, collect cards, play drafts against AI and such, it is a great time.
In the link you can go to releases, there is a installer that is a .jar file, should be called forge-installer.jar or something like that, it installs and in the folder there should be forge.exe and a forge-adventure.exe, the shandalar inspired adventure is the forge-adventure.exe