The requirements for the freelance visa seem quite complex! I've been looking at that as I'm still some years off UK retirement age.
Copied from the Bg goverment website:
Necessary documents
The following shall be attached to the sample application:
(a) a brief description of the type, nature and objectives of the freelance activity;
b) a detailed plan for the activity for the term for which the issuance of a permit for carrying out a freelance activity is requested, compliant with the requirements of Art. 28, para. 2 of PPZTMTM;
c) document / documents, certifying / having the presence of professional experience not less than two years in the activity;
(d) a document establishing the availability of financial resources for carrying out the freelance activity in accordance with the submitted plan;
e) a document for proficiency in the Bulgarian language at level at least B1 of the Common European Framework of Reference for Languages of the Council of Europe;
f) a document that the conditions for exercising a profession included in the List of regulated professions in the Republic of Bulgaria, adopted by a decision of the Council of Ministers on the grounds of Art. 3 of the Law on Recognition of Professional Qualifications;
g) other documents required by the Bulgarian legislation, regulating the respective type of freelance activity, if such are necessary;
(h) a copy of the page of the travel document with the photograph and personal data of the third-country national;
i) two color photos - format 3.5 / 4.5 cm.
In order to continue the permit for carrying out a freelance activity, the third-country national shall submit to the Employment Agency an application in a form, Annex № 4 to the PPZTMTM, with attached:
A) activity plan for the period for which the continuation of the activity is requested;
) at least two service contracts;
C) report on the implementation of the activity plan for the previous period;
D) declaration for absence of obligations for taxes and obligatory insurance contributions;
E) registration certificate from BULSTAT register;
F) patent application under Art. 61n of the Local Taxes Fees Act, when performing patent activities;
(G) a copy of the residence document of the third-country national.
Possibly the biggest challenge, at least for me, will be proof of B1 level proficiency in Bulgarian, which means reasonably fluent. I'm learning, but am not even A1 level yet- basic everyday can-get-by Bulgarian!