Hello drozenrechels,
unfortunately you will find on SciHub no homogenous time-series as you look for. The data you can download are processed with very different Sen2Cor-versions and especially in 2018 there had been major changes. You can find more details in [B. Pflug et.al.,“Next updates of atmospheric correction processor Sen2Cor”, Proc. SPIE 11533, Image and Signal Processing for Remote Sensing XXVI, 1153304 (20 September 2020); https://doi.org/10.1117/12.2574035]. If this is not sufficient don’t hesitate to contact me again.
Data on Google or other archives are just a copy of the data on SciHub. Therefore these archives cannot solve your problem. Reprocessing your data with the latest Sen2Cor version is the only option you have. As always, I recommend to do it not from SNAP, but from command line tool. You have complete control about configuration for your processing and it can run in background at the same time as you are analysing other data in SNAP.
Data archived at SciHub are processed with the same default configuration as is provided with the public Sen2Cor. The only difference is that you cannot use Planet DEM as is used for the products on SciHub. However, you can use SRTM-DEM which should be equivalent for Kenya.
But reprocessing your time series with SRTM-DEM may be a problem because there was a format change in summer. This means, that nothing happens if you have already downloaded the DEM tile with earlier processing. If not, then the only option is waiting for release of Sen2Cor 2.9 which is planned for Q1/2021 actually.
I am wondering that you don’t find the L2A_GIPP.xml configuration file. It should be located in you Sen2Cor home directory.
I hope that I could you help you a little
BdPg