I found a work around that is working for me! This will not solve everyone’s problems, especially if you’re using your Mudita for business. This is also not a de-Googled solution, since RCS is inherently a Google function. However, you can read these messages on a browser on the Mudita without having to install Google services (or any device including iPad/iPhone, computer, etc). I basically tried to copy the relay that Openbubbles uses with an extra iPhone, but using a spare android and Google messages.
My problem: Most of my contacts are using iMessage, so I use Openbubbles on my Mudita and it works GREAT (discussed elsewhere on the forum). I have one large, long distance friend group who is a mix of iPhone and Android users, including some very non-tech savvy folks. I have never received a message from this group to my Mudita. I essentially have lost touch with this group of friends because they use RCS messaging. None of my troubleshooting worked (lots of ideas also discussed elsewhere in the forum). Also, my boyfriend is an Android user and my only regular contact who doesn’t use iMessage. I am frequently unable to see the content of his messages and for some reason the Mudita is forcing SMS and we can’t figure out how to switch to MMS. So in my limited use situation, this work around solves my problems. I have only tested with my boyfriend, but will test the large group soon.
What you need:
-Alternate phone number and pSIM
(Not your primary line for the Mudita, needs to be a dedicated line. I’m using a US mobile multi-network line on warp-verizon for $10/month)
-RCS compatible Android phone with pSIM capability
(Mine is a barely used open box resale for $40, Moto 9G 2025 Android. My model is technically Verizon exclusive but seems to still work for US Mobile warp)
-Google account
(I made a new one just for this purpose, don’t want the RCS stink on my primary number which is linked to my main gmail account)
-Side loaded web browser on your Mudita (If you want/need to be able to read RCS messages on your Mudita. I’m using duck duck go)
Step 1: Activate Google messages on your Android with dedicated pSIM. This will be your RCS/MMS dedicated line. This SIM card has to stay in the phone even while airplane mode, and the phone must be powered on at all times.
Step 2: On the Android, login to the Google account on the Messages app. RCS should be automatically enabled.
Step 3: Login to same Google account on a browser on the device of your choice (Mudita or whatever, I used Mudita and a WiFi only iPhone with no SIM)
Step 4: On your Android, open Messages, and select device pairing and follow the prompts to pair as if you are pairing to a computer, but with the Mudita or whatever device from step 3. More detailed instructions here:
Optional Step 4.5: To ensure all messages default to MMS/RCS (not SMS) when using this workaround switch the Android to airplane mode. This is what I did so I don’t know what happens if you throw SMS in the mix.
Alternate Step 4.5 for US Mobile multi-network line users:
To prevent US Mobile deactivating your line, switch the Android to airplane mode and turn it off, remove primary line pSIM from Mudita, turn off your Mudita, turn Mudita back on, put both primary and multi-network pSIM back in your Mudita, wait a few minutes for US Mobile to register that both the primary line and multi-network line are being used in the same device, then turn off the pSIM slot with the multi-network line, remove the multi-network pSIM, put back into the Android, turn on, but leave in airplane mode (this exact order is important, apparently, according to the US Mobile person I spoke to). I will experiment soon with some deactivated SIM cards to see if they can trick the Android similar to Openbubbles so that the multi-network line can be returned to the Mudita for multi-network data use.
Step 5: Start a group message with your dedicated RCS/MMS line, your primary Mudita line, and the person/people you have issues communicating with. I’m not sure if it matters which device the group message is started from. There is a little bit of a delay with the syncing between devices.
Optional Step 5.5: Make a contact in the Mudita for the RCS/MMS dedicated line called “RCS” or similar so you can easily recognize when you’ve used this work around in the Mudita SMS App.
You should now be receiving all messages for any group text created in this manner to any paired device, without having to install Google services.
This workaround does not affect existing message threads, messages only to your primary line, or group messages created without both the primary line AND the dedicated RCS/MMS line
So far, I have received all MMS messages to my Mudita SMS App as well, but now have the option to open attachments/links on other devices as needed. Hoping that any of these rogue RCS/MMS messages that people are reporting as undelivered on the Mudita SMS App will at least show up in the other devices and in the browser on my Mudita. You can send/receive from any device including the Mudita SMS App and all messages will sync.
Never thought I would have to have two trash phones sitting in a drawer plugged in so that I could keep my Mudita. But if this workaround stands the test of time I’ll finally be a happy Mudita user 
I know this was long but I hope it helps other users feeling the “dumb phone” isolation/FOMO.