Don't think stadium capacity would be that big problem, Eibar played in La Liga for years on a similar stadium. Ipurua's capacity was also less than 6.000 when they were promoted in 2014 and then when they continued to stay up (and were actually a solid midtable team most of the time) they added some seats every summer so they are now at around 8k.
Now if the stadium would fullfil some other requirements it's another question. They would probably need to make at least some small adjustments.
I would expect them to struggle a lot in La Liga having to build a totally new team over the summer. And they would have a month less time than most other La Liga teams (not like teams expected to be in relegation battle were too active on the market so far anyway) due to Segunda playoffs finishing in late June. Their ability to use loans and get the most out of them could help them too. Now they have loanees mostly from Bilbao and Sociedad but in La Liga maybe could even get a few players from a big 3 clubs, Betis or Villarreal. From Barca I think someone like Fort could be a good option for them. I only watched them in these two playoff games but I like how they've used their RB (Hugo Rincon) who is on loan from Bilbao and will probably return there with De Marcos retiring. And in regular season he also had 3 goals and 5 assists so I assume he played well there too. At least offensively.
But I think the biggest problem will be that their coach (Alessio Lisci) has reportedly already agreed to join Osasuna so who knows who will replace him and ofc far from ideal going into La Liga with a new coach and basically totally new squad. According to TM right now they only have one player under contract for next year. Ofc they will probably renew some of the players they owe but still a big rebuild ahead of them regardless of what league they will play in.
