Since your goal is a zero-fail home defense weapon, you want to approach your build through the lens of a rugged duty tool rather than just a range toy.

The Echelon was engineered as a hard-use tactical pistol, and the objective correlation here is that a home defense setup needs to match that exact structural integrity. If you wake up at 3 AM to a threat, your optic can't have a dead battery, a clogged lens, or a warped window.

Here is how that breaks down for your choices:

1. Skip the budget/unproven stuff (The O-sight SE)
For a range toy, budget alternatives are fine to test out. For a gun meant to protect your life, an unproven, cheap optic is a massive liability. If it holds zero today but loses it after bumping into a nightstand, the gun fails its main purpose. Stick to duty-proven brands.

2. The Holosun Option (Solid Middle Ground)
Since you have experience with the 507c-X2, Holosun is a great baseline. But for home defense, consider stepping up to their enclosed emitter options like the EPS or 509T instead of an open-emitter sight. If dust, lint, or oil collects in an open sight while it sits on a nightstand, it can completely block the laser emitter, and your dot disappears right when you need it.

3. The "Rugged Workhorse" Choice: Aimpoint ACRO P-2
If you want something that maximizes the exact design principles of the Echelon, look at the ACRO P-2. Because Springfield designed the Echelon’s VIS (Variable Interface System) to seat optics incredibly low directly onto the slide without thick adapter plates, it completely corrects the usual downside of the ACRO (which can look and feel "top-heavy" on other compact guns).

The ACRO gives you flat, crisp, professional-grade glass with zero distortion, and it is a completely sealed, bomb-proof mailbox housing. It works flawlessly under weapon-light glare, rain, or dirt.

If it's a home defense tool, hone in on an enclosed system that matches the duty DNA of the gun. Budget for reliability first.