https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-savings-contributions-savers-credit There is no $38,000 savers credit cliff, at least not as far as it applies to your situation. If you max out one 401k and one IRA you get the full $2,000 credit up to $63,000 AGI. If you are currently maxing two IRAs and two 401ks, you can cut those contributions in half and still get the full credit (10% of $23,000 is more than $2,000). So the saver's credit does not impact your decision either way. I don't know your state tax situation, but if you remain within that state it is quite possible that state tax savings also do not impact your decision either way.
Let's check farther. We'll generously assume that the $18K you saved at 0% interest went entirely to a 401k and that it precisely lowered your marginal tax rate by 10% (ie. 25% to 15% or 22% to 12%). Let's compare cash-back credit card bonuses from DOC on $18k. Analysis period of one year.
https://www.doctorofcredit.com/best-cash-credit-card-sign-up-bonuses/ and
https://www.doctorofcredit.com/best-bank-account-bonuses/. Includes standard % cash back on spend. Bank bonuses after 12% tax loss.
0% 12 month loan and bank account bonuses$1,800 tax savings (10% on $18,000) (optimistic)
Chase Ink Cash $680 on $18,000 spend, 0% for 12 months
Discover bank bonus $132 on $15,00 for 1 mo
Discover bank bonus $132 on $15,00 for 1 mo (again)
Citi bank bonus $352 on $5,000 for 2 mo
Citi bank bonus $264 on $5,000 for 2 mo
$528 in other bonuses assumed
(Chase has not minimum and thus works wither either strategy and not counted here)
Total Bonus $3,888Credit Card Cash Signup BonusChase Sapphire Preferred: $590 on $4,000 spend
Chase INK Preferred: $955 on $5,000 spend
Chase Ink Cash: $530 on $3,000 spend
Barclaycard CashForward: $220 on $1,000 spend
Chase AARP: $210 on $500 Spend
U.S. Bank Altitude Reserve: $460 on $4,500 spend
Total Bonus: $2,965So you are coming out ahead by $1,000 because of the tax savings, which I assumed worked greatly in your favor with the full amount changing exactly from one bracket to the next. Also many of the credit cards have travel options, gift card options, and other perks which might be far more valuable than cash but I neglected them completely, again in your favor. Also I neglected a 3% fee you mentioned because I assumed the entire amount went to the Chase card. I don't understand 401k loans and ignored costs and risks with those. Finally
You've created enormous unsecured, callable debt
which is a huge risk and not at all comparable to a mortgage.
So you are getting similar or slightly higher returns to the cash-back credit card signup bonus only strategy, but taking more risk in the process. I'm not and expert and these are not the actual numbers, feel free to correct me.