Big Early is highly touted but I have not tried it. Crispy has done well for me. Socrates has been an outstanding performer but not quite as early. Yankee Bell and California Wonder are good OP's. Thin walls are often the result of poor nutrition especially nitrogen and phosphorus. Peppers are more demanding than tomatoes.
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I like Orange Blaze and Gypsy

I've had good luck with good old Cal Wonder, but have grown Better Belle for years and it's been prolific, with firm thick walls. Yes, peppers want more from your soil than tomatoes. I start my own, and found the trick to early production has been an to get an early start on the seedlings, so that they are at least in flower when transplanted. This means bumping them up from cell packs to individual pots whilst still in the greenhouse, but not let get so mature they have a hard time making the transition to the field. Your weather plays into it a lot on early ripening, and I Florida weave my peppers just like I do tomatoes. It seems to have assisted them to get even sun distribution and good fruit set.

I had to look up Lumoyo peppers, which are thick-walled varieties from Spain and Europe. If that's what you want, try 'Vidi' and heed the tips above about adequate fertilizer, water and mulch. The peppers are huge so you only get a few per plant. I also like 'Admiral' yellow bell for stuffing.

If you haven't tried 'Carmen' it should go on your list. Elongated not bell, but really rewarding to grow.

It's not a bell but it tastes like one and is extremely productive and quite early--Corno di Toro. I discovered it some years ago after many years of disappointing bell production.

I haven't had good luck with bell peppers so I seldom grow them but this year II used free seeds. I took seeds from a beautiful yellow bell pepper bought at the supermarket. The two plants I grew had 7 peppers each, one had red and the other yellow. They were thick walled but smaller than the original. They are still flowering so if I had started them sooner I would have gotten more.

Next year I'll try some of the posted suggestions.