This time buy more coins is bad idea, not worth for your hard work and asset. For your safety, just buy bitcoin and eth because the price is low and good to replace your asset. Bitcoin and eth still become the main income for us but remember to use martiangle money management. Need patience this time because I'm afraid down trend still running running until end of the year.
I think martingale strategy is good because it is not gambling anyway so you don't totally lose your money in case the price went down but it would just lose value. Buying those coins that have been existing for a long time s a good strategy because you know that there are demand for those coins as oppose to the new coins that has the tendency to lose demand because the investors nowadays are losing their confidence when it comes to cryptocurrency.
now is the perfect time to buy coins because the market prices are falling but for that you have to be careful in choosing coins because I don't think all coins will give an advantage when the market grows (in the future)
If you want too invest for the long term then choose those coins that has good fundamentals and if possible those coins that are use for its actual use case because you know that there are demand for those. There may be coins that are cheap for now but if those coins are abandoned by the team or there are no progress at all from those project then better look for other coins that has the potential.