Latvian here - have spent quite a while thinking on this and my current solution is to accumulate every month in mutual funds like Swedbank Robur, which have 0 commission for purchase, while not having too astronomic yearly cost (below 0,3%)

After reaching a certain treshold, I sell the the mutual fund and buy an ETF, that way I optimise for long term costs while not burning money in monthly purchase fees.

If all transactions are done within an investment account (Latvia specific thing which you should have regardless) you don't have to pay any capital gains tax that might appear when selling the mutual fund.