1. You don't need premium membership to play premium-only games; you only need 1 person at the table to have a subscription and then everyone else can enjoy the game for free. There are usually tables open looking for players, and if there is no open table to join, free participants can even put out a request for a subscriber to make a new table for strangers to join.
2. You can get 1 month of premium membership at a time for every 100 gift points that you acquire (which gradually expire if left unused). You can gain 5 gift points for winning for your first time at any game, so you can even rig it with someone else to just win at Battleship, Chess, etc. You can also acquire more gift points by contributing to tutorials, wiki entries per game (rules, strategies, etc.), or I think even filing bug reports or improvement ideas per game that get approved by the developers.
3. TTS and Tabletopia are 100% manual so you must already know the rules really well per game, whereas BGA holds your hand through all legal moves, so my circle of friends and I have even used it to learn new games blind (even though they also often have a tutorial). It also has a note system so you can type notes for yourself to remember things between rounds, if you choose to play in the optional turn-based mode (make your move whenever you're next able to, versus real-time in which all players must be online simultaneously).

There's also a digital adaptation of Clank, though I haven't tried it myself. I know the same developers made Root Digital for Leder Games and that adaptation is well-received.