Note that Plastiq is not an IRS-approved payment processor. I would not trust my income tax payments to them, you would not get same protections as those provided by the approved processors. https://www.irs.gov/payments/pay-tax...-or-debit-card

Have you considered paying local property taxes with cards, either to generate spend or use GCs? Individual jurisdictions' practices vary widely, but I would bet that almost all allow partial (split) payments. My jurisdiction charges conv. fee for CCs but not for debit cards if paid in person. They also allow pre-payment of next year's taxes, vs. just what's currently due.