As the regime knew it's intent was to ramp up arrests and deportations, why would they RIF judges, unless their intent was to skip the "due process" part?

Some were just RIF'd as part of the Great Doge Downsizing - they had been recently hired, were within their two-year probationary period, and were just fired along everyone else. Probably thoughtlessly, with few of the DOGE boys worrying too much about whether their services were necessary. The others, who knows? Lots of speculation about whether they were fired for not having the right beliefs/approaches to immigration, but it's difficult to know.

Yes, the practice was to give them a hearing date, and a work permit, so they could support themselves, as it was a lot cheaper than putting them in concentration camps. Reportedly, a favorite place for ICE to grab people is when they make their scheduled check-in with Immigration, or they show up for their court date.

So, why is the regime switching to concentration camps?

I maintain it's the optics of brown people behind barbed wire, for the enjoyment of MAGA.

It was also one of their main campaign promises on immigration. Regardless of whether MAGA wants the optics, immigration hardliners constantly derided the idea of "catch and release." The fact that you had to release these folks after "catch," because there were insufficient beds to detain them for years until their hearing, didn't faze those critics. But now that there's an Administration that is dead set against releasing folks to just be in the community until their hearing date, they have to build more detention camps. They ran on getting all of these folks out of the community, and since they can't deport them without a hearing and lack an effective way of increasing the number of hearings, they need more beds.