Is there any suggested way to modify the rules to play Dominion on your own, like a Dominion Solitaire? It seems possible given that you're mostly just building your own deck and collecting victory points, but I'm not sure how to replicate the scarcity aspect that makes the game competitive and challenging.
Probably the best way to make a bot to play against is to not make it have a deck at all, just every other turn make it take a duchy and on the other turns make it randomly take an attack card and do it's effect (only bother with the effect that effects you for the bot doesn't have a deck) You should check out the game Ascension, it is a deck building game like dominion, but with a viable built in solitaire variant.
I was bored today so I came up with a sort of "Dominion Solitaire".
The way you do it is you set up a standard two player game with you being one player. You simulate the other player's turns in a purely mechanical way, as follows:
Purchase a Province or, if unable to do so, purchase the highest valued treasure you can.
Apart from the randomness in card drawing, you'll be playing a deterministic opponent.
Then see how many points you can beat this "computer" player by.
On Dominion Online, you can play a one-player game. It allows you to do all sorts of solitaire challenges.