Beside the circumstance bonus given by a secondary caster's critical success (or alternatively by the Ritualist Dedication) and permanent magic items the primary caster might have, what are the bonuses that a primary caster could obtain? Also, can consumables be used during a ritual (for example a Serene Mutagen instead of a Primeval Mistletoe) or the ritual must be COMPLETELY uninterrupted?
Yes consumables can be used during a ritual, as long as those consumables have a duration throughout the process.
You could reasonably take a few seconds to drink another elixir as the previous was about to run out to keep the duration going (or another character could administer it to you), if you have enough consumables via something like advanced alchemy.
As with other downtime activities, fortune and misfortune effects can’t modify your checks for the ritual, nor can bonuses or penalties that aren’t active throughout the process. The GM can adjust the DCs of rituals, add or change primary or secondary checks, or even waive requirements to fit specific circumstances. For example, performing a ritual in a location where ley lines converge on the night of a new moon might make a normally difficult ritual drastically easier.
The later points of that section on ritual checks might be a better way to look for bonuses, though much of that is up to the GM and the story they want to tell. There are some rules for ley lines that mostly deal with how they could be used in an encounter rather than downtime, but it does present some information about where to find/where GMs could put ley lines and the nodes where they converge.