I kept my old 401k in the old employer for the reason of protection. 401k's are protected against stuff like some lawsuits, where as ira's are not.
I don't want to roll it to an IRA. and my new 401k isn't as good as the old one, fund choice wise.
in order of operations,
4-6 months living expenses in a high yeild savings account, like marcus.com, or a CD ladder that allows monthly income (eg, you have 12 of them, with 1 coming due every month that covers your monrhtly expenses) This is important to have in case of job loss/etc.
then, focus on savings:
max out the HSA if you have it. it's triple tax advantaged. and don't use it for medical costs unless it's a dire emergency or a big cost.
then 401k to the employer max match %.
assuming you aren't trying to reduce your income for tax purposes, and are within income limits, roth ira.
then resume maxing the 401k to the irs limit.
if you still have left over money, open a taxable brokerage account and invest in an etf that has low roll over for tax purposes (you don't want divs, distributions or cap gains in this account, you want growth stocks!)