The effect of glaciers on the compression/decompression is measurable and is done by using fixed base receivers to collect GPS data that indicate changes in elevation. About 1/4th inch per year in some cases. The sea levels will lower with massive glaciation because of the cycle of water, evaporation, rain. When huge glaciers that covered half of the northern hemisphere formed ocean water evaporated and fell as snow on those glaciers thus locking up water that would otherwise fall back as rain into oceans and their feeder rivers. This also took many millenia but the last ice age from start to finish covered many millenia.
So... how much has the ‘mean’ sea level fallen or risen over the past few millenia?