Why can't I choose Target ROAS as a bidding strategy when creating a standard shopping campaign?
The option to select Target ROAS strategy is not available in the tool, as this strategy (and most bidding strategy types except manual CPC) need to have a certain level of performance data available to work properly. However, you can change the bidding strategy of these shopping campaigns from the Google Ads interface once they get enough performance.
What other dynamic naming formats can be used while creating shopping campaigns, other than "{campaign_name}" and "{ad_group_name}"?
It's not possible to dynamically add any other attribute to the naming convention, at the moment. Only static pieces of text can be added as prefixes or suffixes to the Campaign and the Ad Group naming format.