I don't think there is really enough information to truly diagnose the current cause of the problem. There could be issues with the master cylinder not providing pressure to the caliper, maybe the caliper, maybe alignment of the rotor / caliper. it's hard to say without being there and being able to try to test things out. There are some maintenance kits available for the calipers, and probably for the master cylinder. Maybe a quick rebuild of either or both could be in order, or maybe a replacement. Without knowing more, it's hard to determine.
The biggest unknown is what the true state of the rear wheel and spacers is at this time. It has the wrong wheel and the alignment / positioning / fit of the installed spacers is a bug unknown here. In addition to the issue with the rear brake, misalignment could also result in bearing / hub / wheel damage. maybe clearly visible, maybe not. However, it would only get worse over time.
I'm not a master mechanic by any means, but I did spend about 30 years of my career doing system safety engineering and accident analysis. I think that engrains a serious safety attitude into your mind. Fortunately, we never lost anybody on my watch, but I have seen some things that could have been disastrous if somebody had been in the wrong place. As a rider, I've lost close friends in bike crashes that could have been avoided.
If this bike was in my garage, I would be focused on getting it fixed back the way is was supposed to be, with the correct parts that are in good condition. A rear wheel failure at speed could mean it's your last ride. It's great to save a few bucks unless it causes a crash.
I don't always agree with everybody on the forum, but my priorities on every ride are:
1) Get home safely. No bleeding!
2) Don't get arrested. Keep it sane
3) Have fun.
I want to see you with the opportunity to ride for 50+ years like I have already. It's worth making sure that the bike is fixed right. The rear brake isn't most of your braking power but it certainly makes a difference. It needs to be working properly, and you need to KNOW that the rear wheel is right and safe.
I would like to see you riding and posting here in 20 years, and hopefully I'll be riding as well. Keep safety your priority, and forget the "safe enough... I'll ride differently to compensate" thing. That attitude can end your riding days fast.
That's my $0.02