5
nextSiblingOf() and related methods will only work with Structure entries. What you need instead is next() / prev(): https://docs.craftcms.com/api/v3/craft-base-element.html#next
(At the time of writing, the 3.x docs seem to be lacking any detail on next/prev methods, but the 2.x docs seem to cover it here:
https://craftcms.com/docs/2.x/templating/entrymodel....
3
Some of your steps are unclear, but this should get you started.
{% for category in craft.categories.all() %}
{{ category.title }}
{{ category.url }}
{% set entry = craft.entries
.relatedTo(category)
.orderBy('myField1 ASC')
.one() %}
{% if entry %}
{% set image = entry.myField2.one() %}
{% if image %}
...
1
Typically I use an ids() query to do that kind of thing, as it is usually more efficient than a full-blown .all(), like this:
{% set allPartnerIds = craft.entries.section('partners').limit(null).ids() %}
{% set tiersInUse = craft.categories.group('catPartnerTeirs').relatedTo(allPartnerIds).all() %}
If you have already queried for the partner entries ...
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