While you won't see any changes to your SERP positions or indexing regardless of which path you take here, you're doing the right thing by having internal links open in the same tab. (As @closetnoc pointed out above, including the attribute at all is a bit redundant, since the default behavior of links is to open in the same tab, unless otherwise specified.) ...
A robots meta tag with noindex will prevent Google from indexing a page. The meta tag takes precedence over:
Search console account: Registering for search console will show you additional information about how Google views your site, but it doesn't affect whether or not it is indexed.
Inclusion in an XML sitemap: Including a noindex page in a sitemap ...
SEO is probably the last reason to implement a coding standard. Google has long said that they index sites is they find them, including poorly built sites, as long as they work for users.
In fact, the style guides that Google itself uses prefer smaller HTML for performance over correct HTML that validates. See HTML Style Guide Google vs W3Schools.
you just need to check if the loading time change (TTFB mostly).
Iif it stay the same, your SEO won't be hurt. The technical part on a webpage isn't the major part of the seo algorythm.
Just be sure the structure of your webpage keep the same
using of meta data
H1, H2, Hn structure
Not looking to be a necromancer, but there was a recent article from someone at Yoast that details problems that arise with using the @Graph type when trying to get Google search to "report" the subsequent types, especially with Organization data: link
I would recommend breaking out each type into its own "node" for now instead of using @graph as an array, ...