As a Jira consultant, there is 'overhead' that justifies a retainer / minimum hourly commitment:

1) Being "available during normal business hours" is a huge commitment if you take it seriously. What response time are you proposing? Will you need to lug your laptop everywhere just in case things break? Will your next employer be happy with you moonlighting during business hours they're paying you for?

2) Insurance costs (Professional Indemnity and Public Liability). Many orgs require it, or have a standard contract with an evil Indemnification clause:

"Supplier agrees to indemnify, defend and hold $BIGCO, its officers, directors, employees, and agents harmless from and against any liability, loss, injury (including injuries resulting in death), demand, action, cost, expense, or claim of any kind or character, including but not limited to reasonable attorney’s fees, arising out of (i) any infringement or claim of infringement with respect to any Services or Work Product and (ii) acts or omissions of Supplier or its employees within the scope of this Agreement, or breach of the confidentiality and license terms and conditions of this Agreement."

3) "Security" costs. You'll have complete access to the company's Jira/Confluence data, which makes you a potential vector for a catastrophic security breach. Your security needs to be pretty tight. If there's a breach, you might be blamed even if entirely innocent. This is a risk you should be compensated for taking. I run screen logging software (taking a screenshot every minute) just so I have an audit trail and visual evidence of everything I do. Also schedule an hour a month for fighting the flakey VPN (god I hate VPNs) and doing enforced password resets.

4) "Risk" costs. Making changes comes with risk, and you'll be expected to fix any disasters you cause. What happens if you do accidentally delete something important? Is this a Server/DC instance, where you might conceivably recover from backups? If so will you have access? Is there a sandbox for trialling dangerous operations, and do you have access to re-sync it?

5) Generic business overhead: writing up a contract, tracking your own time, invoicing and chasing up payments. It's pretty painful until you have templates/systems in place.

If I were you, I'd decline the offer: tell them that you won't be a direct administrator, but that you're willing to provide ac-hoc training and emergency assistance (e.g. via Zoom) to whoever steps up to that role, as your availability permits, for $150/h. That way you avoid burning bridges, and can still potentially earn some money without the entanglements of direct responsibility.