Can add javascript to welcome page to add elements and colors; hard to hide fields on welcome page.

NewDocSet.aspx

docsethomepage.aspx (Welcome page) (read only)

EditForm.aspx

Appending text not available in document set libraries, even with versioning turned on.

Make two new edit forms: newdocsetcustom.aspx, editformcustom.aspx

Reference

https://sharepoint.stackexchange.com/questions/30356/customize-newdocset-aspx-form

Create edit page and rename newdocsetcustom.aspx.

if you test, do not click “Preview in Browser”; do not see page.

Edit content type form path in list settings > content type and test from within list.

__WebPartId="{E1EAB40D-6D44-4DFD-B609-A5A6117E15F9} <WebPartPages:ListFormWebPart runat="server" __MarkupType="xmlmarkup" WebPart="true" __WebPartId="{E1EAB40D-6D44-4DFD-B609-A5A6117E15F9}" >
<WebPart xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/WebPart/v2"> <Title>library display name</Title> <FrameType>Default</FrameType> <Description /> <IsIncluded>true</IsIncluded> <PartOrder>2</PartOrder> <FrameState>Normal</FrameState> <Height /> <Width /> <AllowRemove>true</AllowRemove> <AllowZoneChange>true</AllowZoneChange> <AllowMinimize>true</AllowMinimize> <AllowConnect>true</AllowConnect> <AllowEdit>true</AllowEdit> <AllowHide>true</AllowHide> <IsVisible>true</IsVisible> <DetailLink /> <HelpLink /> <HelpMode>Modeless</HelpMode> <Dir>Default</Dir> <PartImageSmall /> <MissingAssembly>Cannot import this Web Part.</MissingAssembly> <PartImageLarge /> <IsIncludedFilter /> <ExportControlledProperties>true</ExportControlledProperties> <ConnectionID>00000000-0000-0000-0000-000000000000</ConnectionID> <ID>g_E1EAB40D_6D44_4DFD_B609_A5A6117E15F9</ID> <ListName xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">{list id}</ListName> <ListId xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">list id</ListId> <PageType xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">PAGE_NEWFORM</PageType> <FormType xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">8</FormType> <ControlMode xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">New</ControlMode> <ViewFlag xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">1048576</ViewFlag> <ViewFlags xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">Default</ViewFlags> <ListItemId xmlns="http://schemas.microsoft.com/WebPart/v2/ListForm">0</ListItemId>
</WebPart>
</WebPartPages:ListFormWebPart> 

Created custom content type with site columns listed in requirements document.

editformcustom.aspx > good

displayformcustom.aspx > good

NewDocSetCustom.aspx > good (created from article at previous link) must customize with javascript selectors; needed to create to populate Form Number field on creation of document set.  

docsethomepage.aspx (Welcome page) (display)  page in general is not customizable except through browser settings and getting element number in dom for css changes.

A. Easiest- Can edit Welcome Page but don’t include fields that need to be hidden.

B. OR you can copy and paste from Display form into docsethomepage.aspx. And set style attribute on element in custom page. However, you will run into navigaton issues with your custom buttons on this page and this will take more time to work out. Easiest and quickest to use the default docsethomepage.aspx.