Hello there! Thanks for your feedback! So overall boots are an essential for Alistar not only for roaming but because he is very position dependent. So high mobility helps a lot by positioning and/or repositioning to pull of engages or roams or even peeling skillshots. So i would heavily recommend, although, your mythic is a massive spike to rush boots for roaming playstyle and even for lane. Tier one boots should be bought really early into the game and then judging by the enemy laners mobility you get to choose if you will continue the boots to tier 2 or go for the mythic.
So, go for the boots if you want to roam as they will help a lot with the timing too.
I hope i covered your question, if not, feel free to contact me again! Have a great day!