Presuming you're working by hand, you would hardly remove much clear coat....quite safe if you do every month. Just that I can't think of any condition whereby you would need to polish every month. Maybe you should try waxing every month and polishing every three months or so instead.

Before this, I used to use dishwashing detergent to wash my mfs.

DG105 is a sealant.
DG501 is an AIO - paint cleaner and sealant.

DG105 don't have much cleaning/polishing properties
DG501 can do a little cleaning/very mild defects removal

DG105 protection last a little longer
DG501 protection last quite decent also, but not as long as DG105

If you're looking for a sealant without any defects/contaminant removing capabilities - DG105
If you're looking for removing some contaminants/very mild defects while putting down a decent layer of protection as well - DG501

If you're undecided - GET BOTH cuz they're both great products and good value for money....hahahaha.....