First and foremost, to build a crypto portfolio might not really have a season or time, although bear market gives more better chances to start cheap. However, you can still take your time to lookout for those low cap coins with good use case and every other thing, keep them in your tabs and wait for a better time to buy; one thing that is certain is, don't chase after a moving coin however be patient with it and you will get a good entry. To add to this, most people might prefer to wait till bear market to buy, and when it comes they will keep waiting while thinking the market will dump more and at the end are left out.
Lastly, bear market like that of 2018 will be hard to come by or experienced, my reason is, the level of adoption today with that of 2018 is quite different and the gap between them is huge, nowadays we keep hearing of many organisations and platforms adopting crypto, we keep seeing many innovations taking place, all these points to a growing market and a growing market like crypto will have it ups and downs but won't get bad like that of previous years.