There are so many factors that could affect repeater performance. First is the quality of the unit. Being your unit is 4 years old, its safe to say the quality is/was above average. Many commercial repeaters (read: Amazon-able) can cause really bad interference issues for RAN networks due to poor quality and poor installation. In your case, it seems to be installed correctly and safe as you've not had a visit from the field engineer or FCC.
From the carrier side, it's not out of the ordinary for physical, mechanical and electrical tilts to be adjusted based on coverage/capacity needs. Much of these tilts, especially electrical, would be unnoticed as they don't require a tower crew. Some of our newer SON tools can manage this functionality automagically based on real time subscriber needs. Even azimuths can be changed based on network needs.
Carrier-management could also be a culprit. Depending on the carrier being used for data flow from the donor site, changes to carrier aggregation methods (ex: 2-way to 4-way MIMO upgrades), band utilization, bandwidth changes, addition of 5G low band aggregation, etc can change the user-experience.
There are so many things that could affect the way your static, directional repeater antenna and 4 year old repeater unit operate. Unfortunately, I only know the operation of the repeater units we deploy. We also don't normally deploy single household repeater units aside from a product we offer, up to BDAs and In-Building small cells. We will rely on our internal network performance information, sourced from all connected handsets in real-time and historically, to make site tilt changes and direct decisions for site upgrades/new site deployments (capacity/coverage).
Hope you get it sorted out.