I'm not a medical professional, just another patient. From what I understand, KRAS is a fairly common mutation that can make the cancer difficult to treat. A large percentage of patients (myself included) with colon cancer have the KRAS mutation. I've read estimates as low as 30% - 40% to as high as "over half". With so many people having the mutation, there's a lot of KRAS treatment research on-going. If you search on PubMed for "KRAS Vitamin C", there are interesting studies about high-dose Vitamin C IVs selectively targeting KRAS and other mutations. You may be able to find other treatment suggestions but I've only focused on high dose Vitamin C for now because it seems to be working for me.
Regarding MSS, it limits your treatment options, at least for now. From what I understand, at this time immunotherapy is not an option for patients with MSS. I don't understand enough to try to explain why. There is also a lot of research on-going in that regard, including trials. I haven't yet looked into any but there's an MSS Trial Finder here: https://fightcolorectalcancer.org/fight/library/dr-toms-late-stage-mss-crc-trial-finder/
I hope this gets you going in the right direction.
Nov-2009 Early stage CRC found during routine colonoscopy
2010, 2011, 2014 F/U colonoscopies, all clear
Jun-2016 CRC during routine follow up colonoscopy, surgery, Stage 4, KRAS, MSS, inoperable lung mets
Aug-2016-May-2018 Folfox, 5FU & Avastin, 5FU, Folfiri & Cyramza
Aug/Sep-2018 YAG laser surgeries (Germany), 11 nodules (9 mets) removed
Nov-2018 clean CT scan
Mar-2019 New lung nodules
Apr-2019 Nov-2020 Xeloda/Avastin, SBRT, continue Xeloda/Avastin
Dec-2020 Progression, considering all options