WIFI on Orbi RBR50/RBS50 network sporadic/slow/dead intermittently after >2y of good performance

Try setting manual channel 1, 6 or 11 on 2.4Ghz

What is the size of your home? Sq Ft?
What is the distance between the router and satellite(s)? 30 feet or more is recommended in between RBR and RBS to begin with depending upon building materials when wirelessly connected.
https://kb.netgear.com/31029/Where-should-I-place-my-Orbi-satellite

One User Experience/Configuration:
https://community.netgear.com/t5/Orbi/Most-Stable-Orbi-Configuration/m-p/1941087/highlight/true#M97026

You can find other speed diagnosis steps in the community faq: https://community.netgear.com/t5/Orbi-AX/Community-FAQ-My-Orbi-speeds-are-slow-inconsistent-and-don-t-my/m-p/1982974/highlight/true#M7849

You might update to more recent FW version thats available;

https://community.netgear.com/t5/Orbi/RBR50-RBS50-Firmware-Version-2-6-1-40/m-p/1997895/highlight/true#M105941

v102 is available if your in the US Region:

https://community.netgear.com/t5/Orbi/Orbi-RBR50-Router-Firmware-Version-V2-7-2-102/m-p/2034115/highlight/true#M110640