Deep ITM Horizontal Debit Call Spread on SPX

Hello, mostly a reader, rarely a poster.

I have been trading options for the past 7-8 years. CCs, spreads mostly.

I was looking for a structure that could use SPX to produce bond-like returns assuming SPX does not drop by some barrier (i.e. 40%), as part of a larger portfolio strategy.

I started looking at calendar debit call spreads 30-50% ITM–Buying perhaps 18 months out and selling 12 months out. Using profit calculators, the returns seem compelling compared to the risks.

I'm hoping to share the considerations I've made, and see if the community is willing to help me understand any risks I've overlooked. I am also very open to resources that might help me learn.

Risks I've considered:

• Bid-ask can be wide, particularly the longer DTE and ITM you go. Even accounting for buying at or close to the ask and selling at or close the bid, returns are compelling to me (12-15% annualized).
• As the short-dated sold call approaches expiry, vega could become substantially lower on the short call compared to the long call. This seems like it could be mitigated substantially by closing the position 30+ DTE, especially if the calls are still substantially ITM. But this is where I don't have experience.
• If I waited until very close to expiration, gamma could go way higher on the short call, especially if SPX price moved lower. Again, seems like it could be mitigated by closing before expiration.
• Could bid ask spread eat profit on the closing side even if I'm happy with opening prices?
• How much would dropping IV impact a deep ITM position? The biggest risk (if closing early) seems to me that the pricing that would fill on the long call would be poor when I wanted to close out.
• Conversely, If IV would impact deep ITM Calendar debit spreads substantially, it seems like a great low-risk way to bet on rising volatility when IV is very low.

Thank you for any thoughts. Greatly appreciate this community even if I don't post much.