  Galaxy clusters are useful cosmological probes and interesting astrophysical
laboratories. With growing cluster samples, a deeper understanding of the
sample characteristics and improved control of systematics becomes more
crucial. In this analysis we create a new and larger ACT-DR5-based thermal
Sunyaev-Zel'dovich Effect (tSZE) selected galaxy cluster catalog with improved
control over sample purity and completeness. We employ the red sequence based
cluster redshift and confirmation tool MCMF together with optical imaging data
from the Legacy Survey DR-10 and infrared data from the WISE satellite to
systematicallyidentify true clusters from a new cluster candidate detection run
on the ACT-DR5 dataset. The resulting ACT-DR5 MCMF sample contains 6,237
clusters with a residual contamination of 10.7%. This is an increase of 51%
compared to the previous ACT-DR5 cluster catalog, making this catalog the
largest tSZE-selected cluster catalog to date. The z>1 subsample contains 703
clusters, three times more than in the previous ACT-DR5 catalog. Matching the
ACT-DR5 MCMF cluster catalog with a deeper tSZE sample from SPTpol 500d allows
us to confirm the completeness and purity of the new ACT-DR5 MCMF sample.
Cross-matching to the two largest X-ray selected cluster samples, the all-sky
RASS MCMF and the half-sky eRASS1, confirms the sample purity of the RASS MCMF
sample and in the case of eRASS1 reveals that 43% of the matched clusters are
designated in eRASS1 as X-ray point sources rather than clusters.
Cross-correlating the ACT-DR5 MCMF cluster catalog with ACT-DR6 lensing maps
results in a 16.4\sigma detection of CMB lensing around the clusters,
corresponding to the strongest signal found so far for a galaxy cluster sample.
Repeating the measurement for the z>1 cluster subsample yields a significance
of 4.3\sigma, which is the strongest CMB lensing detection in a z>1 cluster
sample to date.
