If you really wanted to you can override the handlebars template and change the tabindex value to come last (so it won’t be next after the textarea).
If you add all of this to admin > customize > themes > edit CSS/HTML > header it will do it. Just keep in mind that if we edit our default template you’ll miss out on the changes unless you update this to match it.
This is our default composer template with the reply button set to a tabindex of 9, which makes it last.
<script type="text/x-handlebars" data-template-name="composer">
{{#composer-body composer=model
showPreview=showPreview
openIfDraft=(action "openIfDraft")
typed=(action "typed")
cancelled=(action "cancelled")
save=(action "save")}}
<div class="grippie"></div>
{{#if visible}}
{{composer-messages composer=model
messageCount=messageCount
addLinkLookup=(action "addLinkLookup")}}
{{#if model.viewOpenOrFullscreen}}
<div class="reply-area {{if canEditTags 'with-tags'}}">
<div class='composer-fields'>
{{plugin-outlet name="composer-open" args=(hash model=model)}}
<div class='reply-to'>
{{#unless model.viewFullscreen}}
<div class="reply-details">
{{composer-action-title
model=model
openComposer=(action "openComposer")
closeComposer=(action "closeComposer")
canWhisper=canWhisper
tabindex=8}}
{{plugin-outlet name="composer-action-after" noTags=true args=(hash model=model)}}
{{#unless site.mobileView}}
{{#if isWhispering}}
<span class='whisper'>{{d-icon "far-eye-slash"}}</span>
{{/if}}
{{#if model.unlistTopic}}
<span class='whisper'>({{i18n 'composer.unlist'}})</span>
{{/if}}
{{#if model.noBump}}
<span class="no-bump">{{d-icon "anchor"}}</span>
{{/if}}
{{/unless}}
{{#if canEdit}}
{{#link-to-input onClick=(action "displayEditReason") showInput=showEditReason key="composer.show_edit_reason" class="display-edit-reason"}}
{{text-field value=editReason tabindex="7" id="edit-reason" maxlength="255" placeholderKey="composer.edit_reason_placeholder"}}
{{/link-to-input}}
{{/if}}
</div>
{{/unless}}
{{composer-toggles composeState=model.composeState
toggleComposer=(action "toggle")
toggleToolbar=(action "toggleToolbar")
toggleFullscreen=(action "fullscreenComposer")}}
</div>
{{#unless model.viewFullscreen}}
{{#if model.canEditTitle}}
{{#if model.creatingPrivateMessage}}
<div class='user-selector'>
{{composer-user-selector topicId=topicModel.id
usernames=model.targetUsernames
hasGroups=model.hasTargetGroups
focusTarget=focusTarget
class="users-input"}}
{{#if showWarning}}
<label class='add-warning'>
{{input type="checkbox" checked=model.isWarning tabindex="3"}}
{{i18n "composer.add_warning"}}
</label>
{{/if}}
</div>
{{/if}}
<div class="title-and-category {{if showPreview 'with-preview'}}">
{{composer-title composer=model lastValidatedAt=lastValidatedAt focusTarget=focusTarget}}
{{#if model.showCategoryChooser}}
<div class="category-input">
{{category-chooser
fullWidthOnMobile=true
value=model.categoryId
scopedCategoryId=scopedCategoryId
tabindex="3"}}
{{popup-input-tip validation=categoryValidation}}
</div>
{{/if}}
{{#if canEditTags}}
{{mini-tag-chooser tags=model.tags tabindex="4" categoryId=model.categoryId minimum=model.minimumRequiredTags}}
{{popup-input-tip validation=tagValidation}}
{{/if}}
</div>
{{/if}}
{{plugin-outlet name="composer-fields" args=(hash model=model)}}
{{/unless}}
</div>
{{composer-editor topic=topic
composer=model
lastValidatedAt=lastValidatedAt
canWhisper=canWhisper
storeToolbarState=(action "storeToolbarState")
onPopupMenuAction=(action "onPopupMenuAction")
showUploadModal=(route-action "showUploadSelector")
popupMenuOptions=popupMenuOptions
draftStatus=model.draftStatus
isUploading=isUploading
allowUpload=allowUpload
uploadIcon=uploadIcon
isCancellable=isCancellable
uploadProgress=uploadProgress
groupsMentioned=(action "groupsMentioned")
cannotSeeMention=(action "cannotSeeMention")
importQuote=(action "importQuote")
togglePreview=(action "togglePreview")
showToolbar=showToolbar
afterRefresh=(action "afterRefresh")
focusTarget=focusTarget}}
<div class='submit-panel'>
{{plugin-outlet name="composer-fields-below" args=(hash model=model)}}
<div class='save-or-cancel'>
{{#unless model.viewFullscreen}}
{{composer-save-button action=(action "save")
icon=saveIcon
label=saveLabel
tabindex="9"
disableSubmit=disableSubmit}}
{{#if site.mobileView}}
<a href {{action "cancel"}} class='cancel' tabindex="6" title="{{i18n 'cancel'}}">
{{#if canEdit}}
{{d-icon "times"}}
{{else}}
{{d-icon "far-trash-alt"}}
{{/if}}
</a>
{{else}}
<a href {{action "cancel"}} class='cancel' tabindex="6" >{{i18n 'cancel'}}</a>
{{/if}}
{{/unless}}
{{#if site.mobileView}}
{{#if whisperOrUnlistTopic}}
<span class='whisper'>
{{d-icon "far-eye-slash"}}
</span>
{{/if}}
{{#if model.noBump}}
<span class="no-bump">{{d-icon "anchor"}}</span>
{{/if}}
{{/if}}
{{#if isUploading}}
<div id="file-uploading">
{{loading-spinner size="small"}}<span>{{i18n 'upload_selector.uploading'}} {{uploadProgress}}%</span>
{{#if isCancellable}}
<a href id="cancel-file-upload" {{action "cancelUpload"}}>{{d-icon "times"}}</a>
{{/if}}
</div>
{{/if}}
<div id='draft-status' class="{{if isUploading 'hidden'}}">
{{#if model.draftConflictUser}}
{{avatar model.draftConflictUser imageSize="small"}}
{{/if}}
{{model.draftStatus}}
</div>
</div>
<div class="composer-bottom-right">
{{#if site.mobileView}}
{{#if allowUpload}}
<a class="btn btn-default no-text mobile-file-upload {{if isUploading 'hidden'}}">
{{d-icon uploadIcon}}
</a>
{{/if}}
{{#if showPreview}}
{{d-button action=(action "togglePreview") class="hide-preview" label="composer.hide_preview"}}
{{/if}}
{{else}}
<a href {{action "togglePreview"}} class='toggle-preview'>{{{toggleText}}}</a>
{{/if}}
</div>
</div>
</div>
{{else}}
<div class='saving-text'>
{{#if model.createdPost}}
{{i18n 'composer.saved'}} <a class='permalink' href="{{unbound createdPost.url}}" {{action "viewNewReply"}}>{{i18n 'composer.view_new_post'}}</a>
{{else}}
{{i18n 'composer.saving'}} {{loading-spinner size="small"}}
{{/if}}
</div>
<div class='draft-text'>
{{#if model.topic}}
{{d-icon "share"}} {{{draftTitle}}}
{{else}}
{{i18n "composer.saved_draft"}}
{{/if}}
</div>
{{composer-toggles composeState=model.composeState
toggleFullscreen=(action "openIfDraft")
toggleComposer=(action "toggle")
toggleToolbar=(action "toggleToolbar")}}
{{/if}}
{{/if}}
{{/composer-body}}
</script>