If you are just starting and testing new markets, I would recommend using Shopify Markets. It reduces barriers to cross-border selling and is way cheaper than multi-store approach.
If you have a right budget for expansion and you are very seriuos about it, made your research and already have a quite a lot of customers from targeted countries, then go with multi-store. Expansion through multiple stores presents unparalleled flexibility, offering a more resilient and sustainable solution for internationalization. However, it’s essential to acknowledge that this approach comes with a heftier price tag. The increased costs from additional development time, expenses related to duplicated apps and integrations, and the ongoing management commitment required by this strategy.
In regards to cross border, fulfillment, customs and taxes, have a look at this upcoming webinar as it is pretty relevant: https://global24.clickmeeting.com/cross-border-selling/register