Hi @dhana2 , those are great questions! With respect to historical backfills, you do have the option to do a historical backfill. Brian's posts that I'll link below get into more details here. When possible, I'd recommend adding this historic data to allow for year over year or period over period comparisons.
There's a lot of different thoughts and considerations around using data connector or not.
My recommendation would be to avoid using the data connector when possible. Although it speeds the ingestion of data, it also keeps you in a traditional Analytics framework to an extent. Building out a new schema will require more work and validation, but gives you more flexibility. I would recommend reading two of Brian's recent blog posts for more details.
https://experienceleaguecommunities.adobe.com/t5/adobe-analytics-blogs/how-to-approach-your-first-90-days-with-adobe-customer-journey/ba-p/701089
https://experienceleague.adobe.com/en/docs/analytics-platform/using/compare-aa-cja/upgrade-to-cja/cja-upgrade-getstarted